The number of girls to the number of boys in Primary 2 Faithfulness was 1 : 4. The number of girls to the number of boys in Primary 2 Love was 4 : 5. Both classes had the same number of girls. When 20 boys from Primary 2 Faithfulness joined Primary 2 Love, the ratio of the number of girls to the number of boys in Primary 2 Faithfulness became 2 : 3. Find the total enrolment of both classes.

Primary 2 Faithfulness		Primary 2 Love
Girls	Boys	Girls	Boys
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

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

	Primary 2 Faithfulness		Primary 2 Love
	Girls	Boys	Girls	Boys
Before	4 u	16 u	4 u	5 u
Change		- 20		+ 20
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 20

The number of girls in Primary 2 Faithfulness remains unchanged.  Make the number of girls in Primary 2 the same.  LCM of 4 and 2 is 4.

Number of boys from Primary 2 Faithfulness that joined Primary 2 Love
= 16 u - 6 u
= 10 u

10 u = 20

1 u = 20 ÷ 10 = 2

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 2
= 58

Answer(s): 58