The number of boys is two fifths more than that of girls. The number of boys is a fraction of that of girls. The number of girls is a fraction less than that of boys

The number of boys is two fifths more than that of girls. The number of boys is a fraction of that of girls. The number of girls is a fraction less than that of boys

Suppose that there are x girls and X boys (1. + two fifths) = seven fifths
The number of boys is seven fifths of that of girls
(7 / 5 x-x) divided by 7 / 5 = 2 / 7
Girls are two seventh less than boys