The number of boys to the number of girls in Primary 1 Kindness was 3 : 5. The number of boys to the number of girls in Primary 1 Faithfulness was 4 : 5. Both classes had the same number of boys. When 8 girls from Primary 1 Kindness joined Primary 1 Faithfulness, the ratio of the number of boys to the number of girls in Primary 1 Kindness became 3 : 4. Find the total enrolment of both classes.

Primary 1 Kindness		Primary 1 Faithfulness
Boys	Girls	Boys	Girls
3x4	5x4	4x3	5x3
1x12		1x12
12 u	20 u	12 u	15 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 3 and 4 is 12.

	Primary 1 Kindness		Primary 1 Faithfulness
	Boys	Girls	Boys	Girls
Before	12 u	20 u	12 u	15 u
Change		- 8		+ 8
After	3x4 = 12 u	4x4 = 16 u	12 u	15 u + 8

The number of boys in Primary 1 Kindness remains unchanged.  Make the number of boys in Primary 1 the same.  LCM of 12 and 3 is 12.

Number of girls from Primary 1 Kindness that joined Primary 1 Faithfulness
= 20 u - 16 u
= 4 u

4 u = 8

1 u = 8 ÷ 4 = 2

Total enrolment of both classes
= 12 u + 20 u + 12 u + 15 u
= 59 u
= 59 x 2
= 118

Answer(s): 118