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Chapter 9 · Questions · page 54 of 68

How do you solve a proportion?

Short answerCross multiply, set the two products equal, and divide. For x/6 = 10/15, 15x = 60, so x = 4.

levelPre-algebra
standard7.RP.A.2
practiceProportions

A proportion says two fractions are equal. Multiplying the top of each by the bottom of the other gives equal products, which turns the proportion into a one-step equation.

How to use it

  1. Multiply along each diagonal.
  2. Set the two products equal.
  3. Divide both sides to get the unknown alone.
  4. Check that both fractions reduce to the same value.

Example

x6=1015⇒15x=60⇒x=4\frac{x}{6} = \frac{10}{15} \Rightarrow 15x = 60 \Rightarrow x = 4
35=12n⇒3n=60⇒n=20\frac{3}{5} = \frac{12}{n} \Rightarrow 3n = 60 \Rightarrow n = 20

Where it goes wrong

  • Multiplying straight across instead of along the diagonals.
  • Forgetting the last division step.

Related

Proportions and cross multiplyingRatios and simplest form
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