What is the average of first 21 multiples of 5 ?  Choose the correct option:
A) 70	B) 77	C) 55	D) 81	E) 82

Rationale: required average = 7 ( 1 + 2 + . . . . + 21 ) / 21 ( 5 / 21 ) x ( ( 21 x 22 ) / 2 ) ( because sum of first 21 natural numbers ) = 55 c
Answer: C