PSLE The number of girls and boys in a school hall were in the ratio 2 : 7. After 10.5 girls entered the school hall and half the number of boys left the school hall, the ratio of the number of girls to boys became 1 : 1. How many children were there in the school hall at first?
|
girls |
boys |
Total |
Before |
2 u |
7 u |
9 u |
Change |
+ 10.5 |
- 3.5 u |
|
After |
2 u + 10.5 |
3.5 u |
|
Number of boys who left the school hall
= 7 u ÷ 2
= 3.5 u
Number of girls in the end = 2 u + 10.5
Number of boys in the end = 3.5 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.
3.5 u = 2 u + 10.5
3.5 u - 2 u = 10.5
1.5 u = 10.5
1 u = 10.5 ÷ 1.5 = 7
Total number of children in the school hall at first
= 2 u + 7 u
= 9 u
= 9 x 7
= 63
Answer(s): 63