Sum of first 20 odd natural numbers is 1 + 3 + 5 + 7 + … + 39.

This forms an arithmetic series with first term = a = 1

and common difference = d = 2

Sum of n terms of this arithmetic series is given by:

S_n = [2a + (n - 1)d]

Therefore sum of 20 terms of this arithmetic series is given by:

∴ S₂₀ = [2(1) + (20 - 1)(2)]

= 10 [2 + 38]

= 10 × 40

= 400