Answer :
Boys : Girls = 2:7
let the no. of boys be x
so no of girls = 250+x
so, x:(250+x) = 2:7
x/(250+x) = 2/7
7x = 500+2x
7x-2x = 500
x = 500/5 =100
let the no. of boys be x
so no of girls = 250+x
so, x:(250+x) = 2:7
x/(250+x) = 2/7
7x = 500+2x
7x-2x = 500
x = 500/5 =100