The number of boys to the number of girls in Primary 2 Kindness was 1 : 4. The number of boys to the number of girls in Primary 2 Gentleness was 4 : 5. Both classes had the same number of boys. When 30 girls from Primary 2 Kindness joined Primary 2 Gentleness, the ratio of the number of boys to the number of girls in Primary 2 Kindness became 2 : 3. Find the total enrolment of both classes.

Primary 2 Kindness		Primary 2 Gentleness
Boys	Girls	Boys	Girls
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 1 and 4 is 4.

	Primary 2 Kindness		Primary 2 Gentleness
	Boys	Girls	Boys	Girls
Before	4 u	16 u	4 u	5 u
Change		- 30		+ 30
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 30

The number of boys in Primary 2 Kindness remains unchanged.  Make the number of boys in Primary 2 the same.  LCM of 4 and 2 is 4.

Number of girls from Primary 2 Kindness that joined Primary 2 Gentleness
= 16 u - 6 u
= 10 u

10 u = 30

1 u = 30 ÷ 10 = 3

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 3
= 87

Answer(s): 87