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?
Choose the correct option:
A) 14
B) 15
C) 16
D) 17
E) 18
x + (x+4) + (x+5) + (x+7) = 84   4x + 16 = 84   4x = 68   x = 17   The answer is 17.
<result>D</result>
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?