The number of boys to the number of girls in Primary 4 Joy was 2 : 5. The number of boys to the number of girls in Primary 4 Gentleness was 3 : 4. Both classes had the same number of boys. When 25 girls from Primary 4 Joy joined Primary 4 Gentleness, the ratio of the number of boys to the number of girls in Primary 4 Joy became 3 : 5. Find the total enrolment of both classes.

Primary 4 Joy		Primary 4 Gentleness
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 4 Joy		Primary 4 Gentleness
	Boys	Girls	Boys	Girls
Before	6 u	15 u	6 u	8 u
Change		- 25		+ 25
After	3x2 = 6 u	5x2 = 10 u	6 u	8 u + 25

The number of boys in Primary 4 Joy remains unchanged.  Make the number of boys in Primary 4 the same.  LCM of 6 and 3 is 6.

Number of girls from Primary 4 Joy that joined Primary 4 Gentleness
= 15 u - 10 u
= 5 u

5 u = 25

1 u = 25 ÷ 5 = 5

Total enrolment of both classes
= 6 u + 15 u + 6 u + 8 u
= 35 u
= 35 x 5
= 175

Answer(s): 175