Question

Find the average of first $20$ multiples of $7$

Answer 1

First 20 multiples of 7 are:- $7\times 1,7\times 2,7\times 3,7\times 4,...........,7\times 20$

Required average

$=\frac{7(1+2+3+...+20)}{20}$

$=\frac{7\times 20\times 21}{20\times 2}$ $[\because$ sum of first n numbers$=\frac{n(n+1)}{2}]$

$=\frac{147}{2}=73.5$