Module 6 · Ratios, rates and units · Lesson 25 of 39

Ratio and proportion

Solve a proportion by cross multiplying.

In this lesson · about 14 minutes

  • Write a ratio
  • Solve a proportion for an unknown

A ratio compares two amounts, like 4 to 5, written 45 or 4 : 5. A proportion says two ratios are equal, like 45 = 810.

When one number in a proportion is unknown we call it a variable, usually x, and solve for it. The easiest way is to cross multiply: the numerator of each side times the denominator of the other side are equal. Then divide both sides to get x by itself.

Worked example

Solve 4/5 = 2x/40

Cross multiply, then divide.

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