Mr. Joe’s family consists of six people-himself, his wife and their four children. It is known that the average age of the family immediately after the birth of the first, second, third and fourth child was 16, 15, 16 and 15 respectively. Find the age of Mr. Joe’s eldest son if the present average age of the entire family is 16 years. (a) 8 years    (b) 12 years    (c) 15 years    (d) 16 years