Using Proportions

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Using Proportions
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Ratios

• A ratio is a comparison of two numbers by
  division.
• The SCALE of a blueprint is a ratio of a length
  on the blueprint to the actual length it
  represents.

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Proportions

• A proportion is a statement that two ratios are
  equal.
• In order for two ratios to be equal, their cross
  products must be equal.

• 4 x 35 140 and 5 x 28 140. Therefore
• it is a proportion.

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Solving Proportions

• Use cross products to solve proportions.
• For example,
• Multiply 4 times x and 5 times 3
• 4x -15
• Divide both sides by 4
• X -3.75 or 3 3/4

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Example 1

• On a seven-question science quiz, Stephanie
  answers six questions correctly. Use a
  proportion to find the percent of questions she
  answered correctly.

• Multiply 6 times 100 and 7 times x.
• 7x 600
• Divide both sides by 7.
• X 85.71 about 86.

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Example 2Similar Figures

• Write a proportion comparing corresponding sides
  (sides in the same position).

• Use cross products to solve.
• 8x 30
• Divide both sides by 8.
• X 3.75 cm

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Try these

• A car travels 150 km on 12 L of gas. How many
  liters of gas are needed to travel 500 km?
• An automobile crankshaft revolves 3000 times per
  minute. How long does it take to revolve 50
  times?

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More try these

• Jill tries to guess the number of marbles in an
  8-gallon jar and win a 4-wheeler. She knows there
  are 128 ounces in a gallon, and she finds 46
  marbles fill a 6-ounce jar. How many should she
  guess?

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Try These-Answers

• A car travels 150 km on 12 L of gas. How many
  liters of gas are needed to travel 500 km? 12 L
• An automobile crankshaft revolves 3000 times per
  minute. How long does it take to revolve 50
  times? 1 sec

x 4
y 6/29
x 7 in
11
More try these -answers

• Jill tries to guess the number of marbles in an
  8-gallon jar and win a 4-wheeler. She knows there
  are 128 ounces in a gallon, and she finds 46
  marbles fill a 6-ounce jar. How many should she
  guess?
• 7850 marbles

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Answer
20 cm