The average age of 20 students of a class is 20 years. Out of these the average age of 9 students is 18 years and that of the other 10 students is 21 years. The age of the 20th student is:

19

Total age of 20 students  = (20 × 20)years = 400years

Total age of 9 students  = (9 × 18)years = 162years

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

∴ Age of the 15th student  = 400 − (162 + 219) = 400 − 381 = 19years.