In a school the ratio of boy and girls is 4 : 5 When 100 girls leave the school the ratio become 6 : 7. How many boys are there in the school?

Let number of boys = 4N and number of girls = 5N
Accounting to the question
4N / (5N - 100) = 6/7
⇒ 28N = 6 ( 5N - 100 )
⇒ 28N = 30N - 600
⇒ 2N = 600
∴ N = 600/2 = 300
∴ Number of boys = 4N = 4 x 300 = 1200