Out of four numbers the average of the first three is 16 and that of the last three is 15. If the last number is 21 then the first number is:

28 22 21 24

24

Sum of first three number is = 3 × 16 = 48 Sum of last three number = 3 × 15 = 45 Last number is = 21 Sum of middle two number = 45 - 21 = 24 First number = 48 - 24 = 24