maths ~ f5

let the no. of boys be n
the no. of girls = (1+20%)n
= 1.2n which is an positive integer
.'. n should be divisible by 5.
the no. of students in class = n+1.2n = 2.2n
while 30 < 2.2n <40
30/2.2 < n < 40/2.2
=> 14 < n < 18
n = 15 ('.' n should be divisible by 5.)
the no. of girls in class = 15*1.2 = 18//

No. of girls = 18. (20% more than 15).
No. of boys = 18/1.2 = 15.
Total no. is 18 + 15 = 33 which is more than 30 and less than 40.