The average age of 30 students of a class is 12 years. The average age of a group of 5 of the students is 10 years and that of another group of 5 of them is 14 years. What is the average age of the remaining students?

Sol: Total age of 20 students = (30 x 12) - (5 x 10 + 5 x 14)= 240 years.
Required average = (240/20) = 12 years.