PSLE
The pupils at a camp are divided in Group A and Group B. The ratio of the number of girls to boys in Group A is 7 : 3. The ratio of the number of girls to boys in Group B is 3 : 1. There are 2 times as many pupils in Group B as in Group A. There are 120 more girls in Group B than Group A. What is the total number of boys?

Group A		Group B
10 u		4x5 = 20 u
Girls	Boys	Girls	Boys
7 u	3 u	3x5 = 15 u	1x5 = 5 u

Total number of pupils in Group A
= 7 u + 3 u
= 10 u

Total number of pupils in Group B
= 2 x 10 u
= 20 u

The total number of pupils in Group B is repeated.
Use LCM to make the total number of pupils in Group B the same.
LCM of 20 and 4 is 20.

Number of more girls in Group B than Group A
= 15 u - 7 u
= 8 u

8 u = 120

1 u = 120 ÷ 8 = 15

Total number of boys
= 3 u + 5 u
= 8 u
= 8 x 15
= 120

Answer(s): 120