The number of boys to the number of girls in Primary 1 Goodness was 2 : 5. The number of boys to the number of girls in Primary 1 Patience was 5 : 6. Both classes had the same number of boys. When 20 girls from Primary 1 Goodness joined Primary 1 Patience, the ratio of the number of boys to the number of girls in Primary 1 Goodness became 2 : 3. Find the total enrolment of both classes.

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

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

	Primary 1 Goodness		Primary 1 Patience
	Boys	Girls	Boys	Girls
Before	10 u	25 u	10 u	12 u
Change		- 20		+ 20
After	2x5 = 10 u	3x5 = 15 u	10 u	12 u + 20

The number of boys in Primary 1 Goodness remains unchanged.  Make the number of boys in Primary 1 the same.  LCM of 10 and 2 is 10.

Number of girls from Primary 1 Goodness that joined Primary 1 Patience
= 25 u - 15 u
= 10 u

10 u = 20

1 u = 20 ÷ 10 = 2

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

Answer(s): 114