Math Study Guide: Topic 5: Ratios and Proportions (pages 31-34))

Question 1

When two things are in the ratio 2:3, what does that mean?

Answer 1

It means that for every 2 of the first thing there are 3 of the second thing.

Example: 4 boys and 6 girls would be a ratio of 2:3

Question 2

If the ratio of boys to girls is 2:3, what can you learn about the total number of students from adding the numbers in the ratio?

Answer 2

2 + 3 = 5, which means the total number of students must be a multiple of 5

(also, you know that the number of boys must be a multiple of 2, and the number of girls must be a multiple of 3)

Question 3

The ratio of cars to trucks to minivans is 1:2:3

If there are 72 vehicles, how many minivans are there?

Answer 3

1 + 2 + 3 = 6

Put the minivan ratio (3) over the total (6), and you see that minivans must be ½ of the total:

½ x 72 = 36

Question 4

What is a “proportion?”

Answer 4

A proportion is an equation that has one fraction equaling another fraction.

Example: ½ = 2/4

Question 5

If one value in a proportion is unknown, how do you usually solve for it?

Answer 5

By using “cross-multiplication”

a/b = c/d

Multiply both sides by bd =

ad = bc

Question 6

How would you solve the following?

A cake mix calls for 3 teaspoons of oil for every 5 cups of flour. Jill uses 9 cups of flour. How many teaspoons of oil does she need?

Answer 6

3/5 = x/9 (a/b = c/d)

Cross multiply to get 3 x 9 = 5x =

27/5 = x =

Jill needs 27/5, or 5 2/5 teaspoons of oil