The average age of 15 students of a class is 15 years. Out of these, the average age of 5 students is 12 years and that of the other 9 students is 16 years. The age of the 15th student is? Choose the correct choice:
A) 11 years
B) 17 years
C) 21 years
D) 14 years
E) 12 years

Rationale: "age of the 15 th student = [ 15 * 15 - ( 12 * 5 + 16 * 9 ) ] = ( 225 - 204 ) = 21 years . answer : c"
Answer: C