2.  A ratio is a comparison of two quantities.
 You can write the ratio of a to b or a:b
as the quotient .

a
b

3. Step 1: Write both
measurements in the
same units.
Model: 4 inches
Actual Car: 15 feet = 15 x 12
in. = 180 in.
Step 2: Write the ratio.
We will put the length of the
model in the numerator
since it was listed first.
Step 3: Simplify the ratio. We
can divide 4 and 180 by 4.
So, the ratio of the length of
the scale model to the
length of the car is 1:45
length of model
length of car

4 in
15 ft

4 in
180 in

4
180

1
45

4.  A proportion is
a statement
that two ratios
are equal.
 Examples:
 The Cross-Product Property
states that the product of
the extremes of a
proportion is equal to the
product of the means.

a
b

c
d
a:b c :d

ad bc

5. Example 2:
2
5

n
35
2 35  5 n
70  5n
70
5

5n
5
14  n
Cross-Product
Property
Simplify
Divide each side
by 5

6. Example 3:
y  3
8

y
4
4(y  3)  8y
4y 12  8y
4y  4y
12  4y
12
4

4y
4
3  y
Cross-Product Property
Distribute
Subtract 4y on both sides
Simplify
Divide both sides by 4

7.  In a scale drawing, the scale compares
each length in the drawing to the actual
length.
 The lengths used in a scale can be in
different units.
 A scale might be written as
› 1 in. to 100 mi.
› 1 in. = 12 ft
› 1 mm : 1 m
 You can use proportions to find the actual
dimensions represented in a scale drawing

8. Step 1: Write a
proportion using x for
your unknown length.
 Step 2: Solve the
proportion.

map length (in.)
actual distance (mi.)

1
9

3
x

1
9

3
x
1 x  9 3
x  27
Cross-Product
Property
Simplify
Step 3: Answer the question. Don’t forget units!
The actual distance between the two towns is 27 miles.