What is the average of first 21 multiples of 6 ?	Choose the correct answer
A) 66  B) 77  C) 79  D) 81  E) 82

Rationale: "required average = 6 ( 1 + 2 + . . . . + 21 ) / 21 ( 6 / 21 ) x ( ( 21 x 22 ) / 2 ) ( because sum of first 21 natural numbers ) = 66 a"
Answer: A