The number of girls to the number of boys in Primary 1 Peace was 3 : 4. The number of girls to the number of boys in Primary 1 Gentleness was 5 : 6. Both classes had the same number of girls. When 4 boys from Primary 1 Peace joined Primary 1 Gentleness, the ratio of the number of girls to the number of boys in Primary 1 Peace became 5 : 6. Find the total enrolment of both classes.

Primary 1 Peace		Primary 1 Gentleness
Girls	Boys	Girls	Boys
3x5	4x5	5x3	6x3
1x15		1x15
15 u	20 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 1 Peace		Primary 1 Gentleness
	Girls	Boys	Girls	Boys
Before	15 u	20 u	15 u	18 u
Change		- 4		+ 4
After	5x3 = 15 u	6x3 = 18 u	15 u	18 u + 4

The number of girls in Primary 1 Peace remains unchanged.  Make the number of girls in Primary 1 the same.  LCM of 15 and 5 is 15.

Number of boys from Primary 1 Peace that joined Primary 1 Gentleness
= 20 u - 18 u
= 2 u

2 u = 4

1 u = 4 ÷ 2 = 2

Total enrolment of both classes
= 15 u + 20 u + 15 u + 18 u
= 68 u
= 68 x 2
= 136

Answer(s): 136