The average mark obtained by 15 students was 10 and the average mark obtained by 10 students was What was the average mark obtained by all students?

A 10

B 8

C 12

D 15

Correct Answer: Option C

Total mark of first 15 students = 15 * 10 = 150

Total mark of first 10 students = 15 * 10 = 150

Total mark of 25 students = 150 + 150 = 300

Average = 300 / 25 = 12