There were some boys and girls in the school hall at first. 40% of the boys and 10% of the girls left the school hall. As a result, 3/4 of the pupils remained in the school hall.There were 12 more girls than boys who remained in the school hall. how many boys were there at first?

Answer 1

If there were originally b boys and g girls,

.60b + .90g = .75(b+g)
.9g = .6b+12

solve for b

It's a bit less cumbersome if you get rid of those pesky decimals

60b+90g = 75b+75g
3g = 2b+40

60b + 30(2b+40) = 75b+25(2b+40)
b = 40