The number of girls to the number of boys in Primary 5 Peace was 1 : 4. 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 Peace joined Primary 5 Goodness, the ratio of the number of girls to the number of boys in Primary 5 Peace became 1 : 3. Find the total enrolment of both classes.

Primary 5 Peace		Primary 5 Goodness
Girls	Boys	Girls	Boys
1x5	4x5	5x1	6x1
1x5		1x5
5 u	20 u	5 u	6 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 5 is 5.

	Primary 5 Peace		Primary 5 Goodness
	Girls	Boys	Girls	Boys
Before	5 u	20 u	5 u	6 u
Change		- 10		+ 10
After	1x5 = 5 u	3x5 = 15 u	5 u	6 u + 10

The number of girls in Primary 5 Peace remains unchanged.  Make the number of girls in Primary 5 the same.  LCM of 5 and 1 is 5.

Number of boys from Primary 5 Peace that joined Primary 5 Goodness
= 20 u - 15 u
= 5 u

5 u = 10

1 u = 10 ÷ 5 = 2

Total enrolment of both classes
= 5 u + 20 u + 5 u + 6 u
= 36 u
= 36 x 2
= 72

Answer(s): 72