For a fixed total number of boys and girls, there is only one possible answer. The boys to girls ratio is 3:2, which means 3/5 are boys and 2/5 are girls. If the total number of boys and girls is N, then
number of boys = (3/5)N
number of girls = (2/5)N
Obviously, N must be a multiple of 5, since we cannot have "fractional" boys or girls.