The number of boys to the number of girls in Primary 2 Joy was 2 : 5. The number of boys to the number of girls in Primary 2 Goodness was 3 : 4. Both classes had the same number of boys. When 14 girls from Primary 2 Joy joined Primary 2 Goodness, the ratio of the number of boys to the number of girls in Primary 2 Joy became 3 : 4. Find the total enrolment of both classes.

Primary 2 Joy		Primary 2 Goodness
Boys	Girls	Boys	Girls
2x3	5x3	3x2	4x2
1x6		1x6
6 u	15 u	6 u	8 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 2 and 3 is 6.

	Primary 2 Joy		Primary 2 Goodness
	Boys	Girls	Boys	Girls
Before	6 u	15 u	6 u	8 u
Change		- 14		+ 14
After	3x2 = 6 u	4x2 = 8 u	6 u	8 u + 14

The number of boys in Primary 2 Joy remains unchanged.  Make the number of boys in Primary 2 the same.  LCM of 6 and 3 is 6.

Number of girls from Primary 2 Joy that joined Primary 2 Goodness
= 15 u - 8 u
= 7 u

7 u = 14

1 u = 14 ÷ 7 = 2

Total enrolment of both classes
= 6 u + 15 u + 6 u + 8 u
= 35 u
= 35 x 2
= 70

Answer(s): 70