Let the students in three classes be 2x, 3x and 5x. On increasing the students in each classes by 20, the students will be 2x + 20, 3x + 20 and 5x + 20. Now, according to the question, 2x + 20 : 3x + 20 : 5x + 20 :: 4 : 5 : 7
2x+20
3x+20
=
4
5
10x + 100 = 12x + 80 x = 10 Total number of students before increase were = 2x + 3x + 5x = 10x = 10(10)= 100