The number of girls to the number of boys in Primary 1 Peace was 1 : 4. The number of girls to the number of boys in Primary 1 Goodness was 3 : 5. Both classes had the same number of girls. When 21 boys from Primary 1 Peace joined Primary 1 Goodness, the ratio of the number of girls to the number of boys in Primary 1 Peace became 1 : 3. Find the total enrolment of both classes.

Primary 1 Peace		Primary 1 Goodness
Girls	Boys	Girls	Boys
1x3	4x3	3x1	5x1
1x3		1x3
3 u	12 u	3 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 3 is 3.

	Primary 1 Peace		Primary 1 Goodness
	Girls	Boys	Girls	Boys
Before	3 u	12 u	3 u	5 u
Change		- 21		+ 21
After	1x3 = 3 u	3x3 = 9 u	3 u	5 u + 21

The number of girls in Primary 1 Peace remains unchanged.  Make the number of girls in Primary 1 the same.  LCM of 3 and 1 is 3.

Number of boys from Primary 1 Peace that joined Primary 1 Goodness
= 12 u - 9 u
= 3 u

3 u = 21

1 u = 21 ÷ 3 = 7

Total enrolment of both classes
= 3 u + 12 u + 3 u + 5 u
= 23 u
= 23 x 7
= 161

Answer(s): 161