The number of boys to the number of girls in Primary 4 Joy was 1 : 4. The number of boys to the number of girls in Primary 4 Patience was 5 : 6. Both classes had the same number of boys. When 25 girls from Primary 4 Joy joined Primary 4 Patience, the ratio of the number of boys to the number of girls in Primary 4 Joy became 1 : 3. Find the total enrolment of both classes.

Primary 4 Joy		Primary 4 Patience
Boys	Girls	Boys	Girls
1x5	4x5	5x1	6x1
1x5		1x5
5 u	20 u	5 u	6 u

The number of boys in both classes is the same.  Make the number of boys the same.  LCM of 1 and 5 is 5.

	Primary 4 Joy		Primary 4 Patience
	Boys	Girls	Boys	Girls
Before	5 u	20 u	5 u	6 u
Change		- 25		+ 25
After	1x5 = 5 u	3x5 = 15 u	5 u	6 u + 25

The number of boys in Primary 4 Joy remains unchanged.  Make the number of boys in Primary 4 the same.  LCM of 5 and 1 is 5.

Number of girls from Primary 4 Joy that joined Primary 4 Patience
= 20 u - 15 u
= 5 u

5 u = 25

1 u = 25 ÷ 5 = 5

Total enrolment of both classes
= 5 u + 20 u + 5 u + 6 u
= 36 u
= 36 x 5
= 180

Answer(s): 180