The number of boys to the number of girls in Primary 3 Peace was 3 : 5. The number of boys to the number of girls in Primary 3 Faithfulness was 5 : 6. Both classes had the same number of boys. When 8 girls from Primary 3 Peace joined Primary 3 Faithfulness, the ratio of the number of boys to the number of girls in Primary 3 Peace became 5 : 7. Find the total enrolment of both classes.

Primary 3 Peace		Primary 3 Faithfulness
Boys	Girls	Boys	Girls
3x5	5x5	5x3	6x3
1x15		1x15
15 u	25 u	15 u	18 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 3 and 5 is 15.

	Primary 3 Peace		Primary 3 Faithfulness
	Boys	Girls	Boys	Girls
Before	15 u	25 u	15 u	18 u
Change		- 8		+ 8
After	5x3 = 15 u	7x3 = 21 u	15 u	18 u + 8

The number of boys in Primary 3 Peace remains unchanged.  Make the number of boys in Primary 3 the same.  LCM of 15 and 5 is 15.

Number of girls from Primary 3 Peace that joined Primary 3 Faithfulness
= 25 u - 21 u
= 4 u

4 u = 8

1 u = 8 ÷ 4 = 2

Total enrolment of both classes
= 15 u + 25 u + 15 u + 18 u
= 73 u
= 73 x 2
= 146

Answer(s): 146