Problem: 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 15 th student is ? 
Options: a ) 11 years , b ) 17 years , c ) 21 years , d ) 14 years , e ) 12 years
c