The number of girls to the number of boys in Primary 6 Goodness was 3 : 5. The number of girls to the number of boys in Primary 6 Gentleness was 2 : 5. Both classes had the same number of girls. When 6 boys from Primary 6 Goodness joined Primary 6 Gentleness, the ratio of the number of girls to the number of boys in Primary 6 Goodness became 3 : 4. Find the total enrolment of both classes.

Primary 6 Goodness		Primary 6 Gentleness
Girls	Boys	Girls	Boys
3x2	5x2	2x3	5x3
1x6		1x6
6 u	10 u	6 u	15 u

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

	Primary 6 Goodness		Primary 6 Gentleness
	Girls	Boys	Girls	Boys
Before	6 u	10 u	6 u	15 u
Change		- 6		+ 6
After	3x2 = 6 u	4x2 = 8 u	6 u	15 u + 6

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

Number of boys from Primary 6 Goodness that joined Primary 6 Gentleness
= 10 u - 8 u
= 2 u

2 u = 6

1 u = 6 ÷ 2 = 3

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

Answer(s): 111