The sum of first 20 odd natural numbers is

Answer: Option C

First 20 odd natural numbers are 1, 3, 5, 7, 9, 11, 13, 15, . . . . ., 39
Here a = 1, d = 2, n = 20
$$\begin{array}{rl}& \therefore S_{20}=\frac{n}{2}\left[2a+\left(n-1\right)d\right]\\ & =\frac{20}{2}\left[2\times 1+\left(20-1\right)\times 2\right]\\ & =10\left(2+38\right)\\ & =10\times 40\\ & =400\end{array}$$