Answer :

MeenaMe
proprtions 

3 / 6 = 15 / x

so 15 * 6 = 90

divide by 3 and get 30 boys
Another way at looking at it is, for every 3 girls, there are 6 boys or equal to that is a 1:2 ratio. So if there are 15 girls and twice as many guys, there will be 30 guys