The number of girls to the number of boys in Primary 1 Joy was 1 : 4. The number of girls to the number of boys in Primary 1 Faithfulness was 2 : 5. Both classes had the same number of girls. When 28 boys from Primary 1 Joy joined Primary 1 Faithfulness, the ratio of the number of girls to the number of boys in Primary 1 Joy became 1 : 2. Find the total enrolment of both classes.

Primary 1 Joy		Primary 1 Faithfulness
Girls	Boys	Girls	Boys
1x2	4x2	2x1	5x1
1x2		1x2
2 u	8 u	2 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 2 is 2.

	Primary 1 Joy		Primary 1 Faithfulness
	Girls	Boys	Girls	Boys
Before	2 u	8 u	2 u	5 u
Change		- 28		+ 28
After	1x2 = 2 u	2x2 = 4 u	2 u	5 u + 28

The number of girls in Primary 1 Joy remains unchanged.  Make the number of girls in Primary 1 the same.  LCM of 2 and 1 is 2.

Number of boys from Primary 1 Joy that joined Primary 1 Faithfulness
= 8 u - 4 u
= 4 u

4 u = 28

1 u = 28 ÷ 4 = 7

Total enrolment of both classes
= 2 u + 8 u + 2 u + 5 u
= 17 u
= 17 x 7
= 119

Answer(s): 119