PSLE
The students at a sports meet 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 7 : 8. There are 3 times as many students in Group B as in Group A. There are 21 more girls in Group B than Group A. What is the total number of students?

Group A		Group B
10 u		15x2 = 30 u
Girls	Boys	Girls	Boys
7 u	3 u	7x2 = 14 u	8x2 = 16 u

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

Total number of students in Group B
= 3 x 10 u
= 30 u

The total number of students in Group B is repeated.
Use LCM to make the total number of students in Group B the same.
LCM of 30 and 15 is 30.

Number of more girls in Group B than Group A
= 14 u - 7 u
= 7 u

7 u = 21

1 u = 21 ÷ 7 = 3

Total number of students
= 7 u + 3 u + 14 u + 16 u
= 40 u
= 40 x 3
= 120

Answer(s): 120