1. W RITING AND S OLVING P ROPORTIONS Sports A person burned about 150 calories while skateboarding for 30 minutes. About how many calories would the person burn while skateboarding for 60 minutes? In Example 1, you will use a proportion to answer this question. A proportion is an equation that states that two ratios are equivalent. Algebra =, where b and d are nonzero numbers.

2. E XAMPLE 1 U SING E QUIVALENT R ATIOS Sports A person burned about 150 calories while skateboarding for 30 minutes. About how many calories would the person burn while skateboarding for 60 minutes? In Example 1, you will use a proportion to answer this question. To find the number C of calories the person would burn while skateboarding for 60 minutes, solve the proportion =. Answer: Ask yourself, what number can you multiply by 30 to get 60? The answer is 2. So, multiply 150 x 2 to get 300. So the person would burn about 300 calories while skateboarding for 60 minutes.

3. E XAMPLE 2 S OLVING P ROPORTIONS U SING A LGEBRA Solve the proportion =.

4. E XAMPLE 2 W RITING AND S OLVING A P ROPORTION Setting Up a Proportion There are different ways to set up a proportion. Consider the following problem. Yesterday you bought 8 bagels for $4. Today you want only 5 bagels. How much will 5 bagels cost?

5. W RITING AND S OLVING A P ROPORTION

6. L ESSON 5: S OLVING P ROPORTIONS U SING C ROSS P RODUCTS In the Real World Science At space camp, you can sit in a chair that simulates the force of gravity on the moon. A person who weighs 105 pounds on Earth would weigh 17.5 pounds on the moon. How much would a 60 pound dog weigh on the moon? You’ll find the answer in Example 2. In the proportion, the products 2 6 and 3 4 are called cross products. Notice that the cross products are equal. This suggests the following property.

7. C ROSS P RODUCTS P ROPERTY Cross Products Property Words The cross products of a proportion are equal. Cross Products Property Words The cross products of a proportion are equal.

8. E XAMPLE 1 S OLVING A P ROPORTION U SING C ROSS P RODUCTS Use the cross products property to solve.

9. Science At space camp, you can sit in a chair that simulates the force of gravity on the moon. A person who weighs 105 pounds on Earth would weigh 17.5 pounds on the moon. How much would a 60 pound dog weigh on the moon? E XAMPLE 2 W RITING AND S OLVING A P ROPORTION

10. Penguins At an aquarium, the ratio of rockhopper penguins to African penguins is 3 to 7. If there are 50 penguins, how many are rockhoppers? First, determine the ratio of rockhoppers to total penguins. Then, set up proportion. E XAMPLE 3 W RITING AND S OLVING A P ROPORTION

11. Y OUR T URN In John’s class, the ratio of boys to girls is 5 to 8. If there are 39 students in his class, how many are girls?