## Cross-Multiplying Proportions

### Ratios: Learn

A proportion is an equation which states that two ratios are equal.

When the terms of a proportion are cross multiplied, the cross products are equal. To begin cross-multiplying, first write the ratios in fraction format.

21
70
=
3
10

Next, multiply the numerator of the first ratio by the denominator of the second, and also multiply the denominator of the first ratio by the numerator of the second.

21*10 = 70*3
210 = 210

The example above illustrates how the two ratios are actually equal. But cross-multiplying is most useful when one of the terms of the proportion is unknown. We usually represent unknown numbers with x.

Below is an example of finding an unknown term of a proportion using cross-multiplication.

21
70
=
3
10

10x = 70*3
10x = 210

10x
10
=
210
10

x = 21

### Proportions: Practice

#### What is the missing term?

00:00

Determine the value of x by cross-multiplying.

If
=
then x =

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