The number of boys to the number of girls in Primary 1 Kindness was 1 : 4. The number of boys to the number of girls in Primary 1 Love was 4 : 5. Both classes had the same number of boys. When 12 girls from Primary 1 Kindness joined Primary 1 Love, the ratio of the number of boys to the number of girls in Primary 1 Kindness became 1 : 3. Find the total enrolment of both classes.

Primary 1 Kindness		Primary 1 Love
Boys	Girls	Boys	Girls
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 1 and 4 is 4.

	Primary 1 Kindness		Primary 1 Love
	Boys	Girls	Boys	Girls
Before	4 u	16 u	4 u	5 u
Change		- 12		+ 12
After	1x4 = 4 u	3x4 = 12 u	4 u	5 u + 12

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

Number of girls from Primary 1 Kindness that joined Primary 1 Love
= 16 u - 12 u
= 4 u

4 u = 12

1 u = 12 ÷ 4 = 3

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 3
= 87

Answer(s): 87