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 20, than first number is :

Answer: Option C

Let a, b, c, d are four number
∵ Average of first three number a, b, c = 16
Total of (a + b + c) = 16 × 3 = 48.....(i)
Again
∴ Average of last 3 numbers b, c, d = 15
⇒ Total of (b + c + d) = 15 × 3 = 45.....(ii)
⇒ Subtracting equation (i) - (ii), we get
⇒ (a + b + c) - (b + c + d) = 48 - 45
⇒ a - d = 3
⇒ a = - 20 = 3       [Given d = 20]
⇒ a = 23
⇒ Therefore, first number a = 23