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It is used for fractions.

Let's say you have a fraction 1/2 = 2/4. When you multiply the top (numerator) of the left fraction by the bottom (denominator) of the right fraction it equals the bottom of the left fraction times the top of the right fraction. This is cross multiplying: In this case 1x 4 = 2 x 2.

It is useful if you do not know one of the terms and you need to solve it; for example let's say

3/4 = 9/x

Cross multiply to find 3x = 36 and x = 12

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9y ago

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