PSLE
The number of girls and boys in a classroom were in the ratio 2 : 5. After 6 girls entered the classroom and half the number of boys left the classroom, the ratio of the number of girls to boys became 1 : 1. How many children were there in the classroom at first?

	girls	boys	Total
Before	2 u	5 u	7 u
Change	+ 6	- 2.5 u
After	2 u + 6	2.5 u

Number of boys who left the classroom
= 5 u ÷ 2
= 2.5 u

Number of girls in the end = 2 u + 6
Number of boys in the end = 2.5 u

Since the ratio of the number of girls to boys in the end is 1 : 1, the number of girls and boys in the end is equal.

2.5 u = 2 u + 6
2.5 u - 2 u = 6
0.5 u = 6

1 u = 6 ÷ 0.5 = 12

Total number of children in the classroom at first
= 2 u + 5 u
= 7 u
= 7 x 12
= 84

Answer(s): 84