The number of boys to the number of girls in Primary 1 Faithfulness was 2 : 5. The number of boys to the number of girls in Primary 1 Peace was 4 : 5. Both classes had the same number of boys. When 12 girls from Primary 1 Faithfulness joined Primary 1 Peace, the ratio of the number of boys to the number of girls in Primary 1 Faithfulness became 1 : 2. Find the total enrolment of both classes.

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

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

	Primary 1 Faithfulness		Primary 1 Peace
	Boys	Girls	Boys	Girls
Before	4 u	10 u	4 u	5 u
Change		- 12		+ 12
After	1x4 = 4 u	2x4 = 8 u	4 u	5 u + 12

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

Number of girls from Primary 1 Faithfulness that joined Primary 1 Peace
= 10 u - 8 u
= 2 u

2 u = 12

1 u = 12 ÷ 2 = 6

Total enrolment of both classes
= 4 u + 10 u + 4 u + 5 u
= 23 u
= 23 x 6
= 138

Answer(s): 138