The youngest of 4 children has siblings who are 4, 5, and 7 years older than she is. If the average (arithmetic mean) age of the 4 siblings is 21, what is the age of the youngest sibling?
Answers:
A) 14
B) 15
C) 16
D) 17
E) 18

Rationale: x + ( x + 4 ) + ( x + 5 ) + ( x + 7 ) = 84 4 x + 16 = 84 4 x = 68 x = 17 the answer is d .
Answer: D