The average of six observations is 15. The average of first three among them is twice the average of the last three. What is the sum of the last three observations?

30

Sum of 6 numbers = 6 × 15 = 90

Let Average of 1st 3 numbers = A

Then Average of last 3 numbers = 2A

ATQ ,

3× A + 3 × 2A = 90

9A = 90

A = 10

Sum of first 3 numbers = 3 × 10 =  30