The average age of 10 members of a committee is the same as it was 4 years ago, because an old member has been replaced by a young member.
Find how much younger is the new member ? Choose one:  A) 35  B) 15  C) 40  D) 52  E) 14

Rationale: c let the sum of nine member ( total ) = x and the age of old one = z so its average 4 yrs before = ( x + z ) / 10 . after 4 yrs let z be replaced by y . so now avg = ( x + 4 * 10 + y ) / 10 now ( x + z ) / 10 = ( x + 40 + y ) / 10 so after solving it found z = y + 40 . so old person is 40 yrs older than young one .
Answer: C