The number of boys to the number of girls in Primary 6 Joy was 1 : 4. The number of boys to the number of girls in Primary 6 Goodness was 3 : 5. Both classes had the same number of boys. When 15 girls from Primary 6 Joy joined Primary 6 Goodness, the ratio of the number of boys to the number of girls in Primary 6 Joy became 1 : 3. Find the total enrolment of both classes.

Primary 6 Joy		Primary 6 Goodness
Boys	Girls	Boys	Girls
1x3	4x3	3x1	5x1
1x3		1x3
3 u	12 u	3 u	5 u

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

	Primary 6 Joy		Primary 6 Goodness
	Boys	Girls	Boys	Girls
Before	3 u	12 u	3 u	5 u
Change		- 15		+ 15
After	1x3 = 3 u	3x3 = 9 u	3 u	5 u + 15

The number of boys in Primary 6 Joy remains unchanged.  Make the number of boys in Primary 6 the same.  LCM of 3 and 1 is 3.

Number of girls from Primary 6 Joy that joined Primary 6 Goodness
= 12 u - 9 u
= 3 u

3 u = 15

1 u = 15 ÷ 3 = 5

Total enrolment of both classes
= 3 u + 12 u + 3 u + 5 u
= 23 u
= 23 x 5
= 115

Answer(s): 115