The average age of a group of persons going for picnic is years. Fifteen new persons with an average age of 15 years join the group on the spot due to which their average age becomes 15.5 years. The number of persons initially going for picnic is  Choose the correct option:
A) 5  B) 10  C) 15  D) 40  E) 50

Rationale: solution let the initial number of persons be x . then 16 x + 15 x 15 = 15.5 ( x + 20 ) = 0.5 x = 7.5 x = 15 . answer c
Answer: C