According to proportion, if two sets of given numbers are increasing or decreasing in the same ratio, then the ratios are said to be directly proportional to each other. Proportions are denoted using the symbol "::" or "=".

The two terms ‘b’ and ‘c’ are called ‘means or mean terms’, whereas the terms ‘a’ and ‘d’ are known as ‘extremes or extreme terms.’

Explanation: Our first ratio of the number of girls to boys is \$3 : 5\$, and that of the other is \$4 : 7\$, then the proportion can be written as \$3 : 5\$ :: \$4 : 7\$ or \$3/5\$ = \$4/7\$. Here, 3 and 7 are the extremes, while 5 and 4 are the means.