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.

Cross multiplication is the multiplication of the numerator of the first
ratio by the denominator of the second ratio and the multiplication of the denominator
of the first ratio by the numerator of the second ratio.

21 3
-- = --
70 10
21 * 10 = 70 * 3
210 = 210

If one term of a proportion is not known, cross multiplication can be used to
find the value of that term.

x 3
-- = --
70 10
x * 10 = 70 * 3
10x = 210
10x 210
--- = ---
10 10
x = 21