The number of girls to the number of boys in Primary 2 Joy was 2 : 5. The number of girls to the number of boys in Primary 2 Faithfulness was 5 : 6. Both classes had the same number of girls. When 27 boys from Primary 2 Joy joined Primary 2 Faithfulness, the ratio of the number of girls to the number of boys in Primary 2 Joy became 5 : 8. Find the total enrolment of both classes.

Primary 2 Joy		Primary 2 Faithfulness
Girls	Boys	Girls	Boys
2x5	5x5	5x2	6x2
1x10		1x10
10 u	25 u	10 u	12 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 2 and 5 is 10.

	Primary 2 Joy		Primary 2 Faithfulness
	Girls	Boys	Girls	Boys
Before	10 u	25 u	10 u	12 u
Change		- 27		+ 27
After	5x2 = 10 u	8x2 = 16 u	10 u	12 u + 27

The number of girls in Primary 2 Joy remains unchanged.  Make the number of girls in Primary 2 the same.  LCM of 10 and 5 is 10.

Number of boys from Primary 2 Joy that joined Primary 2 Faithfulness
= 25 u - 16 u
= 9 u

9 u = 27

1 u = 27 ÷ 9 = 3

Total enrolment of both classes
= 10 u + 25 u + 10 u + 12 u
= 57 u
= 57 x 3
= 171

Answer(s): 171