3. The Finding One Method.
Example 1.
If it costs 85p for 5 Mars bars, what is the cost of 3 Mars bars ?
Solution.
Cost of 1 mars bar :
Find the cost of 1 Mars bar.
85 ÷5 = 17 p
Cost of 3 Mars bars. Find the cost of 3 Mars bars.
17 x 3 = 51p
3 Mars bars cost 51p.
4. Example 2.
If you are paid £261 for 18 hours work, how much will you be
paid for 23 hours work ?
Solution.
I hours pay Find the pay for 1 hour .
261 ÷ 18 = £ 14.50
14.5 x 23 = 333.5 Calculate 23 hours pay.
You are paid £333.50 for 23 hours work.
5. Example 3.
It takes a production line 90 hours to make 120 cars. How long does
it take to make 330 cars ?
Solution.
Time for 1 car. Calculate the time to make 1 car .
90 ÷ 120 = 0.75 hours. Note :0.75 hours = 45 minutes .
0.75 x 330 = 247.5 hours Calculate the time to make 330 cars.
It takes 247 hours and 30 minutes to make 330 cars.
6. What Goes In The Box ?
Calculate the following:
(1) It costs £21.06 for a hotel to buy 27 boxes of cereal . How
much will 35 boxes cost at the same price per box ?
£27.30
(2) 35 cars are produced in 87.5 hors by a car factory. How
long will it take to produce 142 cars at the same rate of
working ?
355 hours
(3) If you earn £139.20 for 16 hours of work, what will you
earn for 9 hours work paid at the same rate ?
£78.30
7. The Cross-Multiplication Method .
This method is a more sophisticated way of solving direct
proportion questions but it has two advantages:
• It establishes a very strong routine to solve the problems.
• It makes Inverse Proportion questions easier to handle.
Think back to the Mars Bar problem and answer the following questions:
If you have twice as many Mars Bars what happens to the price ?
The price doubles.
If you have three times as many Mars Bars what happens to the price?
The price trebles.
What about ten times the number of Mars Bars? Ten times the price.
8. Key Point.
Whenever you have two quantities and when one is doubled or trebled
etc the other quantity doubles or trebles etc , then you have
direct proportion .
The problem can then be solved by the method demonstrated in the
following slides.
9. Example 1.
If you are paid £ 190.21 for 23 hours work, then what is your pay
for 17 hours work ?
Solution. Identify the two quantities being
talked about:
Pay Hours
Identify the two amounts that go
190.21 23 together.
P 17 Determine what is being asked for
190.21 23 and what other info’ you have.
=
p 17 Ask yourself “ If the first
quantity is doubled , trebled etc,
23 x p = 17 x 190.21 what happens to the second
17 × 190.21 quantity ? Use arrows..
p=
23 It doubles , trebles etc also.
p = £140.59 for 17 hours.
Cross multiply to solve.
11. Example 3.
Find the area Area Angle
A B of the sector 78.5 360o
100o OAB of this
5cm circle if angle A 100o
O
AOB is 100o 78.5 360
=
A 100
Solution. A = 21.8cm2 ( to 1 d.p)
Find the area of the whole circle.
A = ∏ r2 What angle goes with 78.5cm2 ?
A = 3.14 x 5 x 5 Add the other information:
A = 78.5cm2 If you double treble etc the angle
what happens to the area ?
Now decide on the two
quantities in the problem: Cross multiply and solve:
12. Example 3. Arc Length Angle
Calculate the arc
A length AB on this 75.36 360o
172o circle.
A 172o
O 12cm B
75.36 360
=
A 172
Solution.
A = 36cm (to 1 d.p)
Find the circumference of the circle.
C=2∏r What angle goes with 75.36cm ?
C = 2 x 3.14 x 12 If you double treble etc the angle
what happens to the arc length ?
C = 75.36 cm
Cross multiply and solve:
Now decide on the two
quantities in the problem:
13. What Goes In The Box ? 2
(1) The shadow of a tree 7.4m high is 10.6m . How long will the
shadow of a tree 8.9m high be at the same time ?
12.7m
A B
(2) Calculate the area of the sector AOB . 100o
23cm
461.4cm2 to 1 d.p O
(3) Calculate the arc length AB.
40.1cm to 1 d.p
(4) You get 14 out of 23 in a test. What would you get out of 100 for
the same test score ?
60.9% to 1 d.p
15. Graph Of Ladders Made against Time Taken.
Hours
10
8
6
4
2
0 2 4 6 8 10 12 No of ladders.
16. T
Key Fact.
All Direct Proportion graphs are straight
line graphs passing through the origin:
o L
Look at the table of values again:
Ladders 0 1 2 3 4 5
Hours 0 2 4 6 8 10
If you divide the number of hours by the given number of ladders ,
what do you find each time ? ( Hours divided by ladders)
The answer is 2 every time.
We only need one point on the table to complete the whole table.
17. Complete the tables below given that they form direct proportion graphs:
(1) F 0 1 2 3 4 5
G 0 4 8 12 16 20
(2) D 0 3 6 10 20
E 0 18 24 40 80
(3) R 0 2 7 10 100
W 0 10 35 50 500
(4) W 0 3 7 8 12
M 0 4.5 10.5 12 18
18. What Goes In The Box ? 3
Complete the tables below for direct proportion and draw the graph.
G 0 1 2 3 4 5
H 0 6 12 18 24 30
H
30
20
10
0 1 2 3 4 5 G
