The number of girls to the number of boys in Primary 4 Joy was 1 : 4. The number of girls to the number of boys in Primary 4 Love was 3 : 4. Both classes had the same number of girls. When 18 boys from Primary 4 Joy joined Primary 4 Love, the ratio of the number of girls to the number of boys in Primary 4 Joy became 1 : 3. Find the total enrolment of both classes.

Primary 4 Joy		Primary 4 Love
Girls	Boys	Girls	Boys
1x3	4x3	3x1	4x1
1x3		1x3
3 u	12 u	3 u	4 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 3 is 3.

	Primary 4 Joy		Primary 4 Love
	Girls	Boys	Girls	Boys
Before	3 u	12 u	3 u	4 u
Change		- 18		+ 18
After	1x3 = 3 u	3x3 = 9 u	3 u	4 u + 18

The number of girls in Primary 4 Joy remains unchanged.  Make the number of girls in Primary 4 the same.  LCM of 3 and 1 is 3.

Number of boys from Primary 4 Joy that joined Primary 4 Love
= 12 u - 9 u
= 3 u

3 u = 18

1 u = 18 ÷ 3 = 6

Total enrolment of both classes
= 3 u + 12 u + 3 u + 4 u
= 22 u
= 22 x 6
= 132

Answer(s): 132