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

Primary 4 Peace		Primary 4 Gentleness
Boys	Girls	Boys	Girls
1x3	4x3	3x1	4x1
1x3		1x3
3 u	12 u	3 u	4 u

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

	Primary 4 Peace		Primary 4 Gentleness
	Boys	Girls	Boys	Girls
Before	3 u	12 u	3 u	4 u
Change		- 12		+ 12
After	1x3 = 3 u	3x3 = 9 u	3 u	4 u + 12

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

Number of girls from Primary 4 Peace that joined Primary 4 Gentleness
= 12 u - 9 u
= 3 u

3 u = 12

1 u = 12 ÷ 3 = 4

Total enrolment of both classes
= 3 u + 12 u + 3 u + 4 u
= 22 u
= 22 x 4
= 88

Answer(s): 88