The number of girls to the number of boys in Primary 3 Peace was 2 : 5. The number of girls to the number of boys in Primary 3 Patience was 3 : 5. Both classes had the same number of girls. When 21 boys from Primary 3 Peace joined Primary 3 Patience, the ratio of the number of girls to the number of boys in Primary 3 Peace became 3 : 4. Find the total enrolment of both classes.

Primary 3 Peace		Primary 3 Patience
Girls	Boys	Girls	Boys
2x3	5x3	3x2	5x2
1x6		1x6
6 u	15 u	6 u	10 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 2 and 3 is 6.

	Primary 3 Peace		Primary 3 Patience
	Girls	Boys	Girls	Boys
Before	6 u	15 u	6 u	10 u
Change		- 21		+ 21
After	3x2 = 6 u	4x2 = 8 u	6 u	10 u + 21

The number of girls in Primary 3 Peace remains unchanged.  Make the number of girls in Primary 3 the same.  LCM of 6 and 3 is 6.

Number of boys from Primary 3 Peace that joined Primary 3 Patience
= 15 u - 8 u
= 7 u

7 u = 21

1 u = 21 ÷ 7 = 3

Total enrolment of both classes
= 6 u + 15 u + 6 u + 10 u
= 37 u
= 37 x 3
= 111

Answer(s): 111