SOLVING PROPORTIONS USING CROSS PRODUCTS

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SOLVING PROPORTIONS USING CROSS PRODUCTS

• Section 8.5

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Solving Proportions Using Cross Products
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? Youll find the
answer in Example 2.
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Solving Proportions Using Cross Products
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? Youll find the
answer in Example 2.
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Solving Proportions Using Cross Products
Cross Products Property
The cross products of a proportion
are equal.
Words
4 15 60
Numbers
3 20 60
Algebra
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Solving Proportions Using Cross Products
Solving a Proportion Using Cross Products
Write original proportion.
Cross products property.
2d 9 3
Divide each side by 2.
Simplify.
d 13.5
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Solving Proportions Using Cross Products
Writing and Solving a Proportion
Science To find the weight w of a 60 pound dog
on the moon, as described earlier, write and
solve a proportion using the weight of the
person.
Person
Dog
60
weight on Earth
w
weight on moon
105w 17.5 60
Cross products property.
Divide each side by 105.
w 10
Simplify.
ANSWER
A 60 pound dog would weigh 10 pounds on the moon.
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Solving Proportions Using Cross Products
Writing and Solving a Proportion
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.
SOLUTION
For every 10 penguins, 3 are rockhoppers.
3 50 10r
Cross products property.
Set up a proportion to find the number r of
rockhoppers.
Divide each side by 10.
r
rockhoppers
50
15 r
total penguins
Simplify.
ANSWER
There are 15 rockhoppers at the aquarium.
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Solving Proportions Using Cross Products
Cross Products Property

• Write proportion
• Make sure the bottom numbers represent the same
  item and the top numbers represent the same items.

2. Cross Multiply
3. Divide
4. Write out your answer