The number of boys to the number of girls in Primary 2 Goodness was 1 : 4. The number of boys to the number of girls in Primary 2 Faithfulness was 4 : 5. Both classes had the same number of boys. When 50 girls from Primary 2 Goodness joined Primary 2 Faithfulness, the ratio of the number of boys to the number of girls in Primary 2 Goodness became 2 : 3. Find the total enrolment of both classes.

Primary 2 Goodness		Primary 2 Faithfulness
Boys	Girls	Boys	Girls
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

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

	Primary 2 Goodness		Primary 2 Faithfulness
	Boys	Girls	Boys	Girls
Before	4 u	16 u	4 u	5 u
Change		- 50		+ 50
After	2x2 = 4 u	3x2 = 6 u	4 u	5 u + 50

The number of boys in Primary 2 Goodness remains unchanged.  Make the number of boys in Primary 2 the same.  LCM of 4 and 2 is 4.

Number of girls from Primary 2 Goodness that joined Primary 2 Faithfulness
= 16 u - 6 u
= 10 u

10 u = 50

1 u = 50 ÷ 10 = 5

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 5
= 145

Answer(s): 145