We have the following:
3/4 of 40 = 30 girls, therefore 10 boys
Due each boy receives twice as many balloons as each girl
boys get a multiple of 20 balloons (10 * 2), girls get the same multiple of 30 (30 * 1) balloons.
so
x * (30 + 20) = 220 balloons
x * 50 = 220 balloons
x = 220/50
so the number is 4.4
Boys receive 20 * 4.4 = 88 balloons.
Girls receive 30 * 4.4 = 132 balloons.