A proportion is a statement that indicates that two ratios are equivalent. A proportion can be written in several ways, including 2:4 = 9:18 and 3/7 = 6/14. These two are read as "two is to four as nine is to eighteen," and "three is to seven as six is to fourteen." Proportions equate two fractions, and indicate that they are related by a certain factor. For example, 2:4 is the same as one-half, and 9:18 means one-half. These two ratios are equivalent, so we say that the two ratios are proportional.

In a proportion, the cross product of the ratios are equal. For example, the proportion 3:6 = 4:8 can be rewritten in fraction form, and then if you "cross multiply" you'll see that the cross product is equal:

On the GRE, if you see a proportion question, it'll most likely require you to perform cross multiplication to arrive at some value of a variable. For example, solve for the value of x: