The number of girls to the number of boys in Primary 2 Patience was 1 : 4. The number of girls to the number of boys in Primary 2 Joy was 4 : 5. Both classes had the same number of girls. When 16 boys from Primary 2 Patience joined Primary 2 Joy, the ratio of the number of girls to the number of boys in Primary 2 Patience became 1 : 2. Find the total enrolment of both classes.

Primary 2 Patience		Primary 2 Joy
Girls	Boys	Girls	Boys
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 4 is 4.

	Primary 2 Patience		Primary 2 Joy
	Girls	Boys	Girls	Boys
Before	4 u	16 u	4 u	5 u
Change		- 16		+ 16
After	1x4 = 4 u	2x4 = 8 u	4 u	5 u + 16

The number of girls in Primary 2 Patience remains unchanged.  Make the number of girls in Primary 2 the same.  LCM of 4 and 1 is 4.

Number of boys from Primary 2 Patience that joined Primary 2 Joy
= 16 u - 8 u
= 8 u

8 u = 16

1 u = 16 ÷ 8 = 2

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 2
= 58

Answer(s): 58