The number of boys to the number of girls in Primary 1 Patience was 2 : 5. The number of boys to the number of girls in Primary 1 Love was 4 : 5. Both classes had the same number of boys. When 20 girls from Primary 1 Patience joined Primary 1 Love, the ratio of the number of boys to the number of girls in Primary 1 Patience became 2 : 3. Find the total enrolment of both classes.

Primary 1 Patience		Primary 1 Love
Boys	Girls	Boys	Girls
2x2	5x2	4x1	5x1
1x4		1x4
4 u	10 u	4 u	5 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 2 and 4 is 4.

	Primary 1 Patience		Primary 1 Love
	Boys	Girls	Boys	Girls
Before	4 u	10 u	4 u	5 u
Change		- 20		+ 20
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 20

The number of boys in Primary 1 Patience remains unchanged.  Make the number of boys in Primary 1 the same.  LCM of 4 and 2 is 4.

Number of girls from Primary 1 Patience that joined Primary 1 Love
= 10 u - 6 u
= 4 u

4 u = 20

1 u = 20 ÷ 4 = 5

Total enrolment of both classes
= 4 u + 10 u + 4 u + 5 u
= 23 u
= 23 x 5
= 115

Answer(s): 115