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

Primary 6 Love		Primary 6 Goodness
Boys	Girls	Boys	Girls
2x3	5x3	3x2	5x2
1x6		1x6
6 u	15 u	6 u	10 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 6 Love		Primary 6 Goodness
	Boys	Girls	Boys	Girls
Before	6 u	15 u	6 u	10 u
Change		- 20		+ 20
After	3x2 = 6 u	5x2 = 10 u	6 u	10 u + 20

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

Number of girls from Primary 6 Love that joined Primary 6 Goodness
= 15 u - 10 u
= 5 u

5 u = 20

1 u = 20 ÷ 5 = 4

Total enrolment of both classes
= 6 u + 15 u + 6 u + 10 u
= 37 u
= 37 x 4
= 148

Answer(s): 148