The number of girls to the number of boys in Primary 1 Goodness was 1 : 4. The number of girls to the number of boys in Primary 1 Patience was 3 : 4. Both classes had the same number of girls. When 36 boys from Primary 1 Goodness joined Primary 1 Patience, the ratio of the number of girls to the number of boys in Primary 1 Goodness became 1 : 2. Find the total enrolment of both classes.

Primary 1 Goodness		Primary 1 Patience
Girls	Boys	Girls	Boys
1x3	4x3	3x1	4x1
1x3		1x3
3 u	12 u	3 u	4 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 3 is 3.

	Primary 1 Goodness		Primary 1 Patience
	Girls	Boys	Girls	Boys
Before	3 u	12 u	3 u	4 u
Change		- 36		+ 36
After	1x3 = 3 u	2x3 = 6 u	3 u	4 u + 36

The number of girls in Primary 1 Goodness remains unchanged.  Make the number of girls in Primary 1 the same.  LCM of 3 and 1 is 3.

Number of boys from Primary 1 Goodness that joined Primary 1 Patience
= 12 u - 6 u
= 6 u

6 u = 36

1 u = 36 ÷ 6 = 6

Total enrolment of both classes
= 3 u + 12 u + 3 u + 4 u
= 22 u
= 22 x 6
= 132

Answer(s): 132