The number of boys to the number of girls in Primary 1 Love was 1 : 4. The number of boys to the number of girls in Primary 1 Gentleness was 3 : 4. Both classes had the same number of boys. When 21 girls from Primary 1 Love joined Primary 1 Gentleness, the ratio of the number of boys to the number of girls in Primary 1 Love became 1 : 3. Find the total enrolment of both classes.

Primary 1 Love		Primary 1 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 1 Love		Primary 1 Gentleness
	Boys	Girls	Boys	Girls
Before	3 u	12 u	3 u	4 u
Change		- 21		+ 21
After	1x3 = 3 u	3x3 = 9 u	3 u	4 u + 21

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

Number of girls from Primary 1 Love that joined Primary 1 Gentleness
= 12 u - 9 u
= 3 u

3 u = 21

1 u = 21 ÷ 3 = 7

Total enrolment of both classes
= 3 u + 12 u + 3 u + 4 u
= 22 u
= 22 x 7
= 154

Answer(s): 154