When two ratios are equal, they are said to be in proportion.

To verify whether two ratios are in proportion, we simplify the two ratios first and then we determine whether they are equal or not. If both the simplified ratios are equal, they are said to be in proportion. If the simplified ratios are not equal, then the ratios are not in proportion. We use the symbols " :: " or " = " to denote a proportion.

Consider two ratios in proportion, $\frac{a}{b}$ = $\frac{c}{d}$ , ( a:b :: c:d). Here, we have a x d = c x d.

In a statement of proportion, the first and fourth terms are known as extreme terms and the second and third terms are known as middle terms. Thus, if two ratios are in proportion, the product of the extreme terms = product of the middle terms.

Two ratios are said to be in proportion if they are equal. By definition, a, b, c and d is called a proportion, if a:b = c:d.

Consider the number of boys and girls in a class. Let there be 30 boys and 15 girls.

The ratio of boys to girls = 30:15 = 2:1

The above two ratios are the same. We say that these ratios are in proportion. That is, 60:30 is in proportion to 30:15.

This is written as 60 : 30 :: 30 : 15