Q. The average of three numbers is 7 and that is of first two numbers is 4, then what is the third number?
Answer: 13
Notes: The sum of three numbers is 7 $latex \times$ 3 = 21, and the sum of first two numbers is 2 $latex \times$ 4 = 8 Then the third number is 21 - 8 = 13. Hence option [B] is the right answer.

1