The number of girls to the number of boys in Primary 2 Goodness was 2 : 5. The number of girls to the number of boys in Primary 2 Gentleness was 3 : 5. Both classes had the same number of girls. When 12 boys from Primary 2 Goodness joined Primary 2 Gentleness, the ratio of the number of girls to the number of boys in Primary 2 Goodness became 2 : 3. Find the total enrolment of both classes.

Primary 2 Goodness		Primary 2 Gentleness
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 2 Goodness		Primary 2 Gentleness
	Girls	Boys	Girls	Boys
Before	6 u	15 u	6 u	10 u
Change		- 12		+ 12
After	2x3 = 6 u	3x3 = 9 u	6 u	10 u + 12

The number of girls in Primary 2 Goodness remains unchanged.  Make the number of girls in Primary 2 the same.  LCM of 6 and 2 is 6.

Number of boys from Primary 2 Goodness that joined Primary 2 Gentleness
= 15 u - 9 u
= 6 u

6 u = 12

1 u = 12 ÷ 6 = 2

Total enrolment of both classes
= 6 u + 15 u + 6 u + 10 u
= 37 u
= 37 x 2
= 74

Answer(s): 74