Trial and error will be the fastest method here. Given the movement of pupils involve 20 and 60 pupils, the class size is understood to be quite sizable. So logical starting point is to multiple 3/5 by 100 so it becomes 300 badminton pupils to 500 art pupils. From here, we add 60 to 300 = 360, and less 20 from 500 = 480. 360:480 is the same as 3/4.