The number of girls to the number of boys in Primary 6 Faithfulness was 2 : 5. The number of girls to the number of boys in Primary 6 Kindness was 4 : 5. Both classes had the same number of girls. When 6 boys from Primary 6 Faithfulness joined Primary 6 Kindness, the ratio of the number of girls to the number of boys in Primary 6 Faithfulness became 1 : 2. Find the total enrolment of both classes.

Primary 6 Faithfulness		Primary 6 Kindness
Girls	Boys	Girls	Boys
2x2	5x2	4x1	5x1
1x4		1x4
4 u	10 u	4 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 2 and 4 is 4.

	Primary 6 Faithfulness		Primary 6 Kindness
	Girls	Boys	Girls	Boys
Before	4 u	10 u	4 u	5 u
Change		- 6		+ 6
After	1x4 = 4 u	2x4 = 8 u	4 u	5 u + 6

The number of girls in Primary 6 Faithfulness remains unchanged.  Make the number of girls in Primary 6 the same.  LCM of 4 and 1 is 4.

Number of boys from Primary 6 Faithfulness that joined Primary 6 Kindness
= 10 u - 8 u
= 2 u

2 u = 6

1 u = 6 ÷ 2 = 3

Total enrolment of both classes
= 4 u + 10 u + 4 u + 5 u
= 23 u
= 23 x 3
= 69

Answer(s): 69