2. TO BECOME A NURSE:
YOU HAVE TO
LEARN DRUG
DOSAGE
CALCULATIONS
3. ARE THEY EASY?
ο§ Yes and No
ο§ Everybody wants to teach
you how.
ο§ Everybody teaches you a
different way.
ο§ Everybody says their way is
the best way.
ο§ But no one will say you donβt
need to understand basic
math.
4. A QUESTION TO ASK YOURSELF
Do I want to plug into a
formula?
Or do I want to
really understand?
5. SOME QUOTES FOR THOUGHT
β βStudents who developed logical reasoning skills reported
improved confidence in drug dosage calculations.β
β βFeedback and discussion reveal that confidence in arithmetic
skills can be low even when students are able to pass the end of
semester drug dosage calculation exam.β
β βIt is regarded as acceptable that a formula card or mnemonic
can be used to find the correct dose even though this removes
any requirement for performing the underlying computation.β
β βEmphasize studentsβ innate powers of logical reasoning by
reflection.β
β βTeaching how to get to the answer undermines and undervalues
the studentβs own ability to see their way through the logic.β
β βdeeper learning,β βstepwise logical reasoning skills,β βlearning
consolidated by personal reflection.β
Europepmc.org/article/med/27138475
7. ONE GOOD WAY TO KNOW YOU
ARE REALLY THINKING: SEE
THAT ALL THE WAYS OF DOING
= SAME RESULT.
AND THE WAYS ARE:
ο§ The formula (βdesired over haveβ)
ο§ Proportions
ο§ Unit cancellation
ο§ Reasoning by logical step
AND THE PROS AND CONS ARE:
ο§ The formula (βdesired over haveβ): can be very confusing β formulas stated and explained in
many different ways.
ο§ Proportions: no issues, somewhat an indirect approach but very logical.
ο§ Unit cancellation: virtually always works β no brainer once you understand it.
ο§ Reasoning by logical step: it perhaps can take more time in the short run.
8. TAKE A BASIC PROBLEM
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
And letβs start with the famous
βdesired over haveβ method:
Ordered = βDesiredβ = D in the formula
Available = βHaveβ = H in the formula
βQuantityβ = Q in the formula
βAmountβ = A in the formula
π΄πππ’ππ‘ =
π·ππ ππππ
π»ππ£π
Γ ππ’πππ‘ππ‘π¦ =
π·
π»
Γ π =
4 ππ
2 ππ
π₯ 1 ππ = 2 ππ
(1 is understood)
9. DRAWBACKS TO
βDESIRED OVER HAVEβ
ο§ You will see different versions of the formula, e.g. some will replace Q with V for βvolumeβ
β fine for this particular problem (ml is a volume), but what about when its tablets or
capsules?
ο§ You have to correctly identify the D, H, and Q in each problem to get them plugged in
right.
ο§ You have to memorize the formula.
ο§ The units of the desired and have must match β if they donβt it throws a monkey wrench
into the method: you have to convert the units of either the numerator or denominator so
that they do match, and this must be done outside of the formula itself.
ο§ Finally, this way works in basic drug dosage calculations, but what about IV problems (ml
or drop rate, etc.) β NOPE.
10. LETβS TRY PROPORTIONS
1
5
=
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
2 ππ
1 ππ
=
4 ππ
π₯ ππ
Cross multiplying and solving for the amount x:
2π₯ = 4
2 2
π₯ = 2
So the answer is draw up 2 ml.
NOTE: A shortcut that always works is to
multiply the numbers that are diagonal to
each other and then divide that result by
the 3rd number, as:
4 X 1 = 4 and 4 Γ· 2 = 2
Click to see the
numbers being
cancelled.
11. PROS AND CONS OF PROPORTION
β You do have to think of what is happening to set it up (think: there
are 2 mg in each ml of solution, but I want 4 mg to go into the
patient.
β Remember same units on top and bottom of each ratio.
β Remember how to solve for x.
β Can also be used in more complex problems, but then more than
one proportion may be required. With unit cancellation the extra
proportion(s) are worked right into the setup.
β For example, if unit conversion is necessary, just use another
proportion. For example:
I need to covert 4000 mg to g:
4000 ππ
π₯ π
=
1000 ππ
1 π
4000 X 1 = 4000 and 4000 Γ· 1000 = 4 so 4g
12. HOW ABOUT UNIT
CANCELLATION?
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
4 ππ Γ
1ππ
2 ππ
= 2 ππ 4 ππ
1
Γ
1 ππ
2 ππ
=
4 Γ 1
1 Γ 2
ππ = 2 ππ
Or with everything explicit:
Calculator operation:
4 Γ· 1 X 1 Γ· 2 or ignoring the 1βs: 4 Γ· 2
Alternative calculator operations:
4 X 1 Γ· 1 Γ· 2 or ignoring the 1βs: 4 Γ· 2
4 x 1/(1 X 2) or ignoring the 1βs: 4 Γ· 2
When the units cancel you donβt want and the
unit you do want is left standing, you can be sure
the problem is set up correctly. Multiply key
before any number on the top (if a whole number
it is the top) and divide key before any number
on the bottom = the number part of the answer.
Click to watch the units being cancelled.
13. PROS AND CONS UNIT CANCELLATION
β It works for any problem.
β It is a no brainer β cancel the units, it is set up
correctly.
β The order doesnβt matter; I like to start up with the
simple unit (mg) and then the complex (mg/ml)
ο§ You have to use the calculator correctly β just
remember: on top, multiply, on bottom, divide and OK
TO HIT DIVIDED BY KEY MORE THAN ONCE IN A
ROW.
ο§ Looks funny when you flip the complex unit (from g/ml
to ml/g β JUST MAKE THE UNITS CANCEL).
14. ONE MORE WAY: REASONING
BY LOGICAL STEP
Also known as: look Ma, no format!
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
I think: OK, I have a solution here with 2 mg of drug in every 1 ml.
I need to put 4 mg into the patient. So if every ml has 2 mg, the
number of times 2 goes into the 4 that is wanted would be the
number of ml needed. OR, more simply given the simple
numbers involved in this particular problem: I need 4 g to go into
the patient; each 1 ml has 2 g in it; I need twice as much, so 2 ml.
15. PROS AND CONS OF LOGICAL REASONING?
β You really understand the problem.
β But you do need to have the correct understanding, and sometimes our
understanding can be wrong β but with practice you can be quite confident.
β As for every method we have described:
YOU HAVETO UNDERSTAND BASIC MATH
When using logical reasoning the trick is to decide when to multiply,
divide, even add or subtract. When you know basic math, the correct
decision is intuitive and easy.
16. LETβS LOOK AT SOME
KINDS OF PROBLEMS
We will use unit cancellation to solve them.
ο§ Basic drug dosage problems
β’ Available: mass/volume
β’ Available: mass/tablet
ο§ Dosage by weight
ο§ IV rate calculations
β’ Desired volume/time givenβ wanted in mL/hr
β’ Desired volume/time and drop factor given β wanted in gtts/min
β’ Desired mass/time and available mass/volume given β wanted in mL/hr
ο§ Combination IV problems: e.g. given = desired mass drug/kg patient weight/hour,
patient weight and available mass/volume β wanted in mL/hr
ο§ Other IV problems: calculate total infusion volume or time, adjust manual flow rate
ο§ Amounts of non-injectable fluids
Click to see the units cancel. If you end up on the next slide you
clicked once more than there were cancellations.
17. BASIC DRUG DOSAGE:
AVAILABLE MASS/VOLUME
We did this in our example of 4 mg of Ativan by
injection and available was 2 mg/mL, and we wanted
to know how many mL to inject.
Here is another example (with an added twist of unit conversion needed):
Order: 0.1 g of drug by injection
Available: 30 mg/5 mL solution
Problem: how many mL to draw up
0.1 π Γ
1000 ππ
1 π
Γ
5 ππΏ
30 ππ
= 16.666 β¦ = 16.7 ππΏ
Note: This problem had a number, 5 (not just an understood 1) in the volume. I like to start with the simple unit given (g).
This helps me to βfollow my noseβ through the calculation. The complex unit (mg/mL) is flipped so that the units cancel.
Order doesnβt matter, but if you start with the given 30 mg/5mL fact you need to be sure to write the relation with mL on top
(because that is the unit of your wanted answer β your answer must have mL βon topβ and nothing on the bottom).
18. BASIC DRUG DOSAGE:
AVAILABLE IS
MASS/TABLET
Or capsule or whatever.
Order: 25 mg of drug PO
Available: 50 mg/tablet
Problem: how many tablets to administer
25 ππ Γ
1 π‘πππππ‘
50 ππ
= 0.5 π‘πππππ‘
Note: Looks like we are going to have to split that tablet! Again, note how I start with the simple unit given (mg).
It helps to βfollow my nose,β because I know that unit will have to cancel (I want tablets). If I start with the complex
unit I have to think that tablets (the wanted unit) will have to be on the top, but when I start with mg it simply forces
me to write the complex unit the right way so that the mg unit cancels.
19. DOSAGE BY WEIGHT
Order: 1.5 mg of drug per kg of patient weigh
Given information: patient weighs 74.8 lb
Available: 125 mg/ 2 mL solution
Problem: how many mL to draw up
This brings in a famous conversion fact: 1 kg = 2.2 lb
74.8 ππ Γ
1 ππ
2.2 ππ
Γ
1.5 ππ
1 ππ
Γ
2 ππΏ
125 ππ
= .816 = .82 ππΏ
Note: Starting with the simple unit (lb, the patientβs weight) helps us to βfollow our noseβ canceling units β whatever unit
is on the top (and which is not the unit of our answer) must go on the bottom of the next fraction, so that it cancels. As
with all unit cancellation, you are done when all the units you donβt want cancel, and the unit you want in your answer
remains uncanceled. That is the unit of your answer, and you calculate the number. REMEMBER: numbers on top are
multiplied, numbers on the bottom are divided which may mean repeated use of the division key.
20. IV RATE: VOLUME/TIME IS GIVEN β
CALCULATE ML/HR
The flow rate wanted is in mL/hr. This is the usual case with the
βelectronically regulated infusion pump.β
Order: 250 mL of solution to be infused over (per) the next 120 minutes
Available: the solution
Problem: What is the infusion rate in mL/hr?
250 ππΏ
120 πππ
Γ
60 πππ
1 βπ
=
125 ππΏ
1 βπ
Note: you are only given a complex unit (a βper expressionβ), so you have to start with that. Then you use the conversion
fact 1 hr = 60 min. Note in this case we cancel bottom to top (till now it has all been top to bottom). KEEP YOUR EYE
ON THE UNIT OF YOUR ANSWER: in this case a complex unit with mL on top and hr on the bottom. A conversion fact
is true written either way, but we write it so that min cancel. That way not only does mL remain uncanceled on the top,
but hr remains uncanceled on the bottom, which is where you want it!
= 125 mL/hr
21. IV RATE: VOLUME/TIME IS
GIVEN WITH A DROP FACTOR β
CALCULATE GTT/MIN
The flow rate wanted is gtt/min (gtt = βdropsβ). This is the usual
case with the βmanually regulated IV.β
Order: 1000 mL of fluid in 10 hrs
Drop factor given: 15 gtt/mL
Problem: What is the infusion rate in gtt/min?
1000 ππΏ
10 βππ
Γ
1 βπ
60 πππ
Γ
15 ππ‘π‘
1 ππΏ
=
25 ππ‘π‘
1 πππ
= 25 gtt/min
Again, note where you are going: you want gtt on top and min on bottom in the answer. We didnβt have to write the
order or the drop factor flipped β we simply made sure both hours and mL cancel out and we are left with gtt on top and
min on bottom. We cancel both bottom to top and top to bottom as needed. Note that the beginning complex unit (the
order) has mL, a volume, on the top because our answer needs to have gtt, a volume, on the top.
22. IV RATE: MASS/TIME IS
GIVEN β CALCULATE ML/HR
Order: 20 mg of drug per hour
Available: 500 mg in 250 mL of solution
Problem:What is the infusion rate in mL/hr?
250 ππΏ
500 ππ
Γ
20 ππ
1 βπ
=
10 ππΏ
1 βπ
= 10 mL/hr
Note: we want mL on top in the answer, so we write the mass and volume relation with volume on
the top β mg (which is not in the answer) cancels bottom to top.
23. IV PROBLEM: COMBO
WITH PATIENT WEIGHT
Order: 6 mcg/kg/hr
Patient weight: 100 kg
To infuse: 12.5 mg drug in 250 mL of fluid
Problem: What is infusion rate in mL/hr?
100 ππ Γ
6 πππ
ππ Γ βπ
Γ
1 ππ
1000 πππ
Γ
250 ππΏ
12.5 ππ
=
12 ππΏ
βπ
= 12 mL/hr
Note:
6 πππ
ππ
βπ
1
=
6 πππ
ππ
Γ
1
βπ
=
6 πππ
ππ Γβπ
Note: Start with the simple unit (kg) which needs to
cancel. The given per/per order of mass of drug/patient
weight/time is written with the patient weight unit
multiplied by the time unit in the denominator. The mass
of drug per volume relation is written with volume on the
top, because volume is on the top of the wanted answer.
24. MORE VARIATIONS ON IV
PROBLEMS β SAME METHOD
WORKS FINE
Infuse 500 mL at 20 gtt/min, factor = 20 gtt
per mL. What is the total time to infuse?
500 ππΏ Γ
20 ππ‘π‘
ππΏ
Γ
1 πππ
20 ππ‘π‘
Γ
1 βπ
60 πππ
= 8.333β¦ hr = 8 1/3 hr
Infuse the solution over 8 hr at 21 gtt/min, factor
= 10 gtt/mL. What is the total volume needed?
8 βπ Γ
60 πππ
βπ
Γ
21 ππ‘π‘
πππ
Γ
1 ππΏ
10 ππ‘π‘
= 1008 mL
The order was 500 mL over 5 hrs, factor = 10 gtt/mL. After 1 hr of infusion, 300 mL remain in the bag.
Calculate the adjusted flow rate. Note, just a new problem with 300 mL over 4 hr.
300 ππΏ
4 βπ
Γ
1 βπ
60 πππ
Γ
10 ππ‘π‘
ππΏ
= 12.5 gtt/min = 13 gtt/min (rounding to full drop).
Click to cancel.
Click to cancel.
Click to cancel.
25. CALCULATE AMOUNTS OF
NON-INJECTABLE FLUIDS
Ordered: wound care q2h for 3 da using 45 mL of 25%
solution (available is full strength solution = βmedicationβ).
Problem: find the volume of medication and volume of
normal saline to mix to prepare enough.
First calculate total volume needed:
3 ππ Γ
12 πππ ππ
ππ
Γ
45 ππΏ
πππ π
= 1620 mL
Then calculate the volume of medication in that total volume:
1620 ππΏ Γ .25 = 405 ππΏ ππ πππππππ‘πππ
Finally, subtract volume of medication from the total volume to get the volume of normal saline:
1620 ππΏ β 405 ππΏ = 1215 ππΏ ππ ππππππ π πππππ
27. CONVENTIONS PER
DEPARTMENT PRACTICE TESTS
ο§ Answers over 1 mL: round to nearest tenth
ο§ Answers under 1 mL: round to nearest hundredth
ο§ All flow rates are rounded to the nearest whole number
ο§ Decimals less than 1: always put a leading zero (e.g., 0.75
mL)
ο§ Partial tablets written as decimal (e.g., 1.5 tablets)
ο§ READ THE QUESTION: sometimes it will also tell you how
to round.
TIPS:
ο§ βAvailableβ is the same as βOn hand.β
ο§ Donβt type units of your answer if the units are already there after the blank to fill in.
ο§ Note that βdoseβ is the order and is usually in mg.
ο§ βVolumeβ can be used to indicate any kind of answer, not just mL, but also tablets or capsules.
β’ Capsules canβt be divided; a fractional answer or an answer more than three indicate a
possible error.
β’ Tablets can be divided (at least if βscored,β and an answer would usually range from Β½ to 3.
ο§ NOTE: The mass/volume relations come from reading the label. Write so that units cancel.
28. Order: 50 mEq PO daily
Available: 25 mEq/5 mL
Wanted: how many mL to administer
ANSWER: 10 mL
50 ππΈπ Γ
5 ππΏ
25 ππΈπ
= 10 ππΏ
NOTE: The βPO dailyβ is not information used in the calculation. You just need to calculate how
many mL to draw for each dayβs dose. Start with the simple unit (mEq) which you donβt want in
your answer, and write the mEq/volume (which is given) so that the units cancel. You want mL
for your answer, so it goes on top.
βΊIn all problems click to see answer, set
up, see units cancel, and note on set up. If
you end up on next slide you clicked one
too many times.
29. Order: 1.5 g PO 4X daily
Available: 500 mg scored tablets
Wanted: amount to administer
ANSWER: 3 tablets
1.5 π Γ
1000 ππ
1 π
Γ
1 π‘πππππ‘
500 ππ
= 3 π‘πππππ‘π
NOTE: The βPO 4X dailyβ is not used in the calculation. Always interpret the βamount to
administerβ as per dose, unless specifically asked for the daily total or some other total. Start
with simple unit (g) which needs to be converted to mg (because the tablets are given in mg).
That cancels g. Then write the mass per tablet so that the mg also cancels (your answer has
the units βtabletsβ). βTabletsβ is left standing and that is the unit of your answer.
30. Order: 1.4 mg/kg of patient weight
Patient weight 34 lb
Available: 3 mg/mL
Wanted: volume to draw
ANSWER: 34.9066.., round to 34.9 mL
34 ππ Γ
2.2 ππ
1 ππ
Γ
1.4 ππ
1ππ
Γ
1 ππΏ
3 ππ
= 34.90666 β¦ = 34.9 ππΏ
NOTE: A perfect βfollow your noseβ example. Start with the simple unit (lb). That unit must go in
the denominator to cancel. Then with each relation you have (knowing lb to kg is 1lb = 2.2 kg)
write so that the unit on top cancels top to bottom, until what you want, mL, is left standing.
31. Order: 100 mL to be infused every 15
min for 5 hours
Wanted: infusion pump rate in mL/hr
ANSWER: 400 mL/hr
100 ππΏ
15 πππ
Γ
60 πππ
1 βπ
=
400 ππΏ
1 βπ
= 400 ππΏ/βπ
NOTE: Some problems start with a βcomplex unit,β in this case mL/min. We want mL/hr for an answer,
so we write the relation with mL on top. We donβt want min, we want hr, so we make min cancel bottom
to top. That leaves us with mL on top and hr on the bottom for our mL/hr answer. We didnβt use the
total time of 5 hours.
But what if the order stated more simply: βinfuse 2000 mL over 5 hours.β Then: 2000 mL divided by 5 hr = 400
mL/hr. Sometimes the answer is just reducing the fraction given!
32. Order: 1500 mL to infuse over 12 hr
Given: drop factor = 20 gtt/mL
Wanted: manual IV infusion rate in gtt/min
ANSWER: 42 gtt/min
1500 ππΏ
12 βπ
Γ
1 βπ
60 πππ
Γ
20 ππ‘π‘
1 ππΏ
=
42 ππ‘π‘
1 πππ
= 42 ππ‘π‘/πππ
NOTE: There is no simple unit to start with, because 1500 mL over 12 hr is a rate of mL/hr. We start
with that, because that is volume over time which our answer will also be (just different units). Then
we use known and given relations to convert hr to min and mL to gtt (order does not matter, just so
what we donβt want cancels, and what we do want remains: gtt on top, and min on bottom).
(41.666β¦ = 42)
33. Order: infuse a solution of 2g in 500 mL at 4mg/min.
Wanted: flow rate for infusion pump in mL/hr
ANSWER: 60 mL/hr
500 ππΏ
2 π
Γ
1 π
1000 ππ
Γ
4 ππ
1 πππ
Γ
60 πππ
1 βπ
=
60 ππΏ
1 βπ
= 60 ππΏ/βπ
NOTE: No simple unit to start with, so we start with the solution relation with mL on top (for the top of our
answer). Then in any order, conversion of g to mg, cancel mg with the given mass to time rate, and conversion
of min to hr. However, I always use the natural order: whatever unit that is in what I have just written (a unit I
donβt want in my answer), I cancel it with the next thing I write. Sometimes one of the units may have to wait to
be cancelled by something later in the series. In this case, units always cancel bottom to top immediately.
34. Order: infuse a solution of 5 g in 1000 mL at a rate
of 1 mg/kg/hour
Patient weight: 50 lb
Wanted: flow rate of infusion pump in mL/hr
50 ππ Γ
2.2 ππ
1 ππ
Γ
1 ππ
ππ Γ βπ
Γ
1 π
1000 ππ
Γ
1000 ππΏ
5 π
=
22 ππΏ
1 βπ
= 22ππΏ/βπ
ANSWER: 22 mL/hr
NOTE: We do have a simple unit to start with, the patientβs weight in lb. We first convert to kg and
then use the βper/perβ expression so that kg cancels, then since our solution relation has g, we
convert to grams (cancelling the mg). Finally we write our solution relation with mL at the top (where
it is in the answer). Grams then cancels, and note the hr we want in our answer is still on the bottom
uncancelled. In this case all cancellations are top to bottom.
35. Order: 900 mL to infuse at 150 mL/hr
Wanted: if we start at 0730, when will the
infusion be done?
ANSWER: 1330
900 ππΏ Γ
1 βπ
150 ππΏ
= 6 βπ
0730 + 6 hr = 1330
Military time: after 1200, instead of going back
to 0100 (which means 1 AM) we just keep
counting: 1 PM =1300, 8 PM = 2000, 11 PM =
2300, 12 midnight = 0000. For afternoon time
just add 12: 2 PM + 12 = 1400.
NOTE: The set up is wonderfully simple for this problem. Start with the simple unit (mL) and cancel the
mL which you donβt want leaving the hr that you do. Then, for a change, we ADD to get the ending
military time.
36. Order: 8.45% NS to infuse at 125 mL/hr over 32 hr
Wanted: the total volume needed
ANSWER: 4000 mL
32 βπ Γ
125 ππΏ
βπ
= 4000 ππΏ
NOTE: Again the setup is really simple. Start with that simple unit (hr) and then look at your rate which
has hours in it (and the mL you need). Make the hr cancel and youβre done.
37. Order: Infuse 50 mL of solution in 15 min
Drop factor: 15 gtt/mL
Wanted: infusion rate (manual IV) in gtt/mL
ANSWER: 50 gtt/min
50 ππΏ
15 πππ
Γ
15 ππ‘π‘
ππΏ
=
50 ππ‘π‘
1 πππ
= 50 ππ‘π‘/πππ
Problem: After 5 min there are 40 mL left in the IV bag.
Wanted: Adjusted flow rate so that the drug is finished
infusing on time.
ANSWER: 60 gtt/mL
40 ππΏ
10 πππ
Γ
15 ππ‘π‘
ππΏ
=
60 ππ‘π‘
1 πππ
= 60 ππ‘π‘/πππ
NOTE: We start with mL on the top, because that is a volume and we want a volume (gtt) on top in
our answer. We only need to cancel mL. For the adjusted flow rate problem, we of course have to
SUBTRACT 15 β 5 = 10 min remaining to put in the given (now) 40 mL.
38. Order: For wound care β 1 Β½ oz of ΒΎ strength wound care solution applied
each hr for 24 hr.
Available: full strength solution (βmedicationβ)
Wanted: volume of medication and volume of normal saline to prepare enough
ANSWER: 810 mL of medication and 270 mL of normal saline
Calculate total volume needed:
24 πππ ππ Γ
1.5 ππ§
πππ π
Γ
30 ππΏ
1 ππ§
= 1080 ππΏ
Calculate the volume of medication needed:
3
4
Γ 1080 ππΏ = 810 ππΏ
Calculate the volume of normal saline needed:
1080 mL β 810 mL = 270 mL
NOTE: Start with the simple unit (doses), then use the
oz per dose, then convert oz to mL (memorize that
conversion). The strength of the solution is best done
as a separate calculation (the ΒΎ or .75 does not have
units). Finally, add or subtract needs to be done
separately from unit cancellation: unit cancellation is
only multiply and divide.
39. REVIEW: IMPORTANCE OF
CORRECT CALCULATOR
OPERATION
EXAMPLE:
74.8 ππ Γ
1 ππ
2.2 ππ
Γ
1.5 ππ
1 ππ
Γ
2 ππΏ
125 ππ
= .816 = .82 ππΏ
DO:
74.8 X 1 Γ· 2.2 X 1.5 Γ· 1 X 2 Γ· 125 = .816 or 74.8 Γ· 2.2 X 1.5 X 2 Γ· 125
OR
74.8 X 1 X 1.5 X 2 Γ· 2.2 Γ· 1 Γ· 125 or 74.8 X 1.5 X 2 Γ· 2.2 Γ· 125
DO NOT DO:
74.8 X 1 X 1.5 X 2 Γ· 2.2 X 1 X 125 = 8500 or 74.8 X 1.5 X 2 Γ· 2.2 X 125 = 8500
Unless you use PARENTHESES:
74.8 X 1 X 1,5 X 2 Γ· (2.2 X 1 X 125) = .816 or 74.8 X 1.5 Γ· (2.2 X 125) = .816
REMEMBER: It is fine to
use Γ· two or more times in a
row. Just be sure any
number on top gets X and
any number on the bottom
gets Γ· .
40. WRAP UP
β Memorize formulas β not recommended.
β Proportion β OK, but on more complicated problems (IV) may take 2 or 3 separate ones.
β Unit cancellation β you canβt go wrong.
o However: always check to see if the unit (uncancelled) that you end up with is the unit of the
wanted answer β otherwise you stopped too soon!
o Always look toward the units of the answer: what is on top of the answer will have to be
somewhere on top of the set-up (and not cancel), and what is on the bottom of the answer will
have to be somewhere on the bottom of the set-up (and not cancel).
o If a number has a single unit only β that is, it is simply a whole number with a unit, no fraction
involved (like lb or whatever), it is understood to be on the top. You may write it over a 1 if that
helps you.
o It helps to start with a simple unit number if possible, but sometimes you have to start with a
βcomplexβ unit which is a ratio (βrateβ) or βper expression.β For example:
50 mL
15 min
. Just be sure
what you want on the top of your answer is on the top of the ratio.
ο Try it! I did an hour practice test in 45 min, and I found myself using ALL unit cancellation. I got,
oops, 90%, but that was from 3 simple errors. One I stopped to soon (check the units when youβre
done!). One I put .75 instead of 0.75. And one was a rounding error (up instead of down).
ο GO FOR IT! PRACTICE! ALL THE BEST IN YOUR NURSING CAREER!
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