The number of girls to the number of boys in Primary 3 Gentleness was 2 : 5. The number of girls to the number of boys in Primary 3 Goodness was 5 : 6. Both classes had the same number of girls. When 22 boys from Primary 3 Gentleness joined Primary 3 Goodness, the ratio of the number of girls to the number of boys in Primary 3 Gentleness became 5 : 7. Find the total enrolment of both classes.

Primary 3 Gentleness		Primary 3 Goodness
Girls	Boys	Girls	Boys
2x5	5x5	5x2	6x2
1x10		1x10
10 u	25 u	10 u	12 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 2 and 5 is 10.

	Primary 3 Gentleness		Primary 3 Goodness
	Girls	Boys	Girls	Boys
Before	10 u	25 u	10 u	12 u
Change		- 22		+ 22
After	5x2 = 10 u	7x2 = 14 u	10 u	12 u + 22

The number of girls in Primary 3 Gentleness remains unchanged.  Make the number of girls in Primary 3 the same.  LCM of 10 and 5 is 10.

Number of boys from Primary 3 Gentleness that joined Primary 3 Goodness
= 25 u - 14 u
= 11 u

11 u = 22

1 u = 22 ÷ 11 = 2

Total enrolment of both classes
= 10 u + 25 u + 10 u + 12 u
= 57 u
= 57 x 2
= 114

Answer(s): 114