took sometime to understand the question. I did the pick number way, then saw how the algebra worked, lol.

so pick a mid number, x=225
90 of boys = 225% --> 90/b = 2.25
so b = 90/2.25, b = 40

boys make up 40% of total population X
so b/x = 40% --> 40/225 = 17%, NO

so not C. 17% is too low, to increase it, X has to be smaller. Try B

x = 150
90/b = 150, b = 60
60/150 = 40%, YES

Algebraically (from back solving)

90/boys = x%
boys/x (total population) = 40%
x can be solved

the best way to approach such problem is go the way it says....

90=x/100 of boys --------1

boys = 40/100 of x---------2

Put 2 in 1 we get

90=x/100 * 40/100 * x

solve this.. u get 22500=x^2

thus x=150

HTH