The number of girls to the number of boys in Primary 5 Joy was 2 : 5. The number of girls to the number of boys in Primary 5 Goodness was 3 : 4. Both classes had the same number of girls. When 18 boys from Primary 5 Joy joined Primary 5 Goodness, the ratio of the number of girls to the number of boys in Primary 5 Joy became 2 : 3. Find the total enrolment of both classes.

Primary 5 Joy		Primary 5 Goodness
Girls	Boys	Girls	Boys
2x3	5x3	3x2	4x2
1x6		1x6
6 u	15 u	6 u	8 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 5 Joy		Primary 5 Goodness
	Girls	Boys	Girls	Boys
Before	6 u	15 u	6 u	8 u
Change		- 18		+ 18
After	2x3 = 6 u	3x3 = 9 u	6 u	8 u + 18

The number of girls in Primary 5 Joy remains unchanged.  Make the number of girls in Primary 5 the same.  LCM of 6 and 2 is 6.

Number of boys from Primary 5 Joy that joined Primary 5 Goodness
= 15 u - 9 u
= 6 u

6 u = 18

1 u = 18 ÷ 6 = 3

Total enrolment of both classes
= 6 u + 15 u + 6 u + 8 u
= 35 u
= 35 x 3
= 105

Answer(s): 105