The youngest of 4 children has siblings who are 3, 5, and 8 years older than she is. If the average (arithmetic mean) age of the 4 siblings is 20, what is the age of the youngest sibling?
Choose the correct answer:
A) 14 B) 15 C) 16 D) 17 E) 18

Rationale: "x + ( x + 3 ) + ( x + 5 ) + ( x + 8 ) = 80 4 x + 16 = 80 4 x = 64 x = 16 the answer is c ."
Answer: C