The average of 7 numbers is 15, that of the first three is 6 and that of the last three is 23. What is the fourth number?

• A. 15
• B. 7
• C. 13
• D. 18

Answer: (D)

4th No. = (Total of 7 Nos) - (Total of first three + total of last 3)             = (15 x 7) - (6 x 3 + 23 x 3)             = 105 - (18 + 69)             =105 - 87             = 18