Proportions

Simple Ratio problems can be solved with a proportion. For example:

The ratio of girls to boys in the class is 4 to 7. If there are 35 boys in the class, how many girls are there?

Step 1: Set up a labeled proportion:

Step 2: Cross-multiply to solve:

Remember that you can cancel fractions to simplify calculations as you go ( = ), although you can usually avoid this extra step by using the calculator. For example:

Note: Never cancel factors diagonally across an equals sign. That would change the values incorrectly.

Check Your Skills

1. The ratio of apples to oranges in a fruit basket is 3 : 5. If there are 15 apples, how many oranges are there?

2. Miki has 7 jazz CDs for every 12 classical CDs in his collection. If he has 60 classical CDs, how many jazz CDs does he have?