The number of girls to the number of boys in Primary 5 Joy was 2 : 5. The number of girls to the number of boys in Primary 5 Kindness was 4 : 5. Both classes had the same number of girls. When 20 boys from Primary 5 Joy joined Primary 5 Kindness, the ratio of the number of girls to the number of boys in Primary 5 Joy became 2 : 3. Find the total enrolment of both classes.

Primary 5 Joy		Primary 5 Kindness
Girls	Boys	Girls	Boys
2x2	5x2	4x1	5x1
1x4		1x4
4 u	10 u	4 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 2 and 4 is 4.

	Primary 5 Joy		Primary 5 Kindness
	Girls	Boys	Girls	Boys
Before	4 u	10 u	4 u	5 u
Change		- 20		+ 20
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 20

The number of girls in Primary 5 Joy remains unchanged.  Make the number of girls in Primary 5 the same.  LCM of 4 and 2 is 4.

Number of boys from Primary 5 Joy that joined Primary 5 Kindness
= 10 u - 6 u
= 4 u

4 u = 20

1 u = 20 ÷ 4 = 5

Total enrolment of both classes
= 4 u + 10 u + 4 u + 5 u
= 23 u
= 23 x 5
= 115

Answer(s): 115