PSLE The number of boys and girls in a school hall were in the ratio 2 : 7. After 9 boys entered the school hall and half the number of girls left the school hall, the ratio of the number of boys to girls became 1 : 1. How many children were there in the school hall at first?
|
boys |
girls |
Total |
Before |
2 u |
7 u |
9 u |
Change |
+ 9 |
- 3.5 u |
|
After |
2 u + 9 |
3.5 u |
|
Number of girls who left the school hall
= 7 u ÷ 2
= 3.5 u
Number of boys in the end = 2 u + 9
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 + 9
3.5 u - 2 u = 9
1.5 u = 9
1 u = 9 ÷ 1.5 = 6
Total number of children in the school hall at first
= 2 u + 7 u
= 9 u
= 9 x 6
= 54
Answer(s): 54