Let B be the number of boys and G= number of girls:
B/G = 2/7 (given)
And
G= 250+B (given), then
B/G = B/(250+B) = 2/7
B/(250+B) = 2/7 → Cross multiplication:
7.B = 2(250+B) → 7.B = 500 + 2.B
5.B = 500 And B = 500/5 And B = 100
Boys = 100 and Girls = 350
Proof: 100/350 = 10/35 = 2/7