The sum of first 20 odd natural numbers is:$( A) \ 100$
$( B) \ 210$
$( C) \ 400$
$( D) \ 420$

Given: First 20 odd numbers.

To do: To find out the sum of first 20 odd numbers.

Solution: First 20 odd numbers are $1,\ 3,\ 5,\ 7,\ \dotsc \dotsc \dotsc \dotsc ,\ 39$

It is an A.P., here first term $a=1$, last term $l=39$ and common difference $d=2$

We know sum of an A.P., $S_{n}=\frac{n}{2} \ ( a+l) =\frac{20}{2}( 1+39) =\frac{20\times 40}{2} =400\ $

$\therefore$ Option $( C)$ is correct.