The average age of 25 students of a class is 20 years. Out of these the average age of 10 students is 21 years and that of the other 14 students is 18 years. The age of the 25th student is:

38

Total age of 25 students  = (25 × 20)years = 500years

Total age of 10 students  = (10 × 21)years = 210years

Total age of other 14 students  = (14 × 18)years = 252years

∴ Age of the 25th student  = 500 − (210 + 252) = 500 − 462  = 38years.