Aptitude Hack#44(8-4-19)

Question:
What is the average of the first 21 multiples of 7?
A) 49
B) 77
C) 66
D) 57

Answer: B) 77

Explanation:

Required average  = 7(1+2+....+21) / 21
=> (7/21)X ( (21x22) / 2 )  (because sum of first 21 natural numbers => (n*n+1)/2 )
= 77