The number of girls to the number of boys in Primary 5 Faithfulness was 3 : 5. The number of girls to the number of boys in Primary 5 Goodness was 5 : 6. Both classes had the same number of girls. When 10 boys from Primary 5 Faithfulness joined Primary 5 Goodness, the ratio of the number of girls to the number of boys in Primary 5 Faithfulness became 3 : 4. Find the total enrolment of both classes.

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

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

	Primary 5 Faithfulness		Primary 5 Goodness
	Girls	Boys	Girls	Boys
Before	15 u	25 u	15 u	18 u
Change		- 10		+ 10
After	3x5 = 15 u	4x5 = 20 u	15 u	18 u + 10

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

Number of boys from Primary 5 Faithfulness that joined Primary 5 Goodness
= 25 u - 20 u
= 5 u

5 u = 10

1 u = 10 ÷ 5 = 2

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

Answer(s): 146