The number of boys to the number of girls in Primary 1 Joy was 2 : 5. The number of boys to the number of girls in Primary 1 Gentleness was 4 : 5. Both classes had the same number of boys. When 24 girls from Primary 1 Joy joined Primary 1 Gentleness, the ratio of the number of boys to the number of girls in Primary 1 Joy became 2 : 3. Find the total enrolment of both classes.

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

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

	Primary 1 Joy		Primary 1 Gentleness
	Boys	Girls	Boys	Girls
Before	4 u	10 u	4 u	5 u
Change		- 24		+ 24
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 24

The number of boys in Primary 1 Joy remains unchanged.  Make the number of boys in Primary 1 the same.  LCM of 4 and 2 is 4.

Number of girls from Primary 1 Joy that joined Primary 1 Gentleness
= 10 u - 6 u
= 4 u

4 u = 24

1 u = 24 ÷ 4 = 6

Total enrolment of both classes
= 4 u + 10 u + 4 u + 5 u
= 23 u
= 23 x 6
= 138

Answer(s): 138