PSLE The number of boys and girls in a canteen were in the ratio 2 : 7. After 15 boys entered the canteen and half the number of girls left the canteen, the ratio of the number of boys to girls became 1 : 1. How many children were there in the canteen at first?
|
boys |
girls |
Total |
Before |
2 u |
7 u |
9 u |
Change |
+ 15 |
- 3.5 u |
|
After |
2 u + 15 |
3.5 u |
|
Number of girls who left the canteen
= 7 u ÷ 2
= 3.5 u
Number of boys in the end = 2 u + 15
Number of girls in the end = 3.5 u
Since the ratio of the number of boys to girls in the end is 1 : 1, the number of boys and girls in the end is equal.
3.5 u = 2 u + 15
3.5 u - 2 u = 15
1.5 u = 15
1 u = 15 ÷ 1.5 = 10
Total number of children in the canteen at first
= 2 u + 7 u
= 9 u
= 9 x 10
= 90
Answer(s): 90