Question

The sum of first 20 odd natural numbers is:

• A. 100
• B. 210
• C. 400
• D. 420

Answer 1

The correct option is C 400

Odd natural numbers are in the pattern 1,3,5,7,9.....

These numbers form an A.P. where a = 1, d = 3 1 = 2

We know that, $S_n=[2a+(n-1\left)d\right]$

$\therefore S_{20}=\frac{20}{2}[2\times 1+(20-1)\times 2]=\frac{20}{2}[2+19\times 2]$
=10[2+38]
$=10\times 40$
=400

Thus, the sum of first 20 odd natural numbers is 400.