PSLE The number of girls and boys in a classroom were in the ratio 1 : 5. After 4.5 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 |
1 u |
5 u |
6 u |
Change |
+ 4.5 |
- 2.5 u |
|
After |
1 u + 4.5 |
2.5 u |
|
Number of boys who left the classroom
= 5 u ÷ 2
= 2.5 u
Number of girls in the end = 1 u + 4.5
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 = 1 u + 4.5
2.5 u - 1 u = 4.5
1.5 u = 4.5
1 u = 4.5 ÷ 1.5 = 3
Total number of children in the classroom at first
= 1 u + 5 u
= 6 u
= 6 x 3
= 18
Answer(s): 18