The sum of first 20 odd natural numbers is:

First 20 odd natural numbers:
First term, a = 1
Common difference, d = 2
Sum of first n terms of an AP: n/2(2a + (n-1)d)
Here, n = 20, a = 1, d = 2
Substituting these, we get
Sum of first 20 odd natural numbers:
20/2(2(1) + (20-1)2) = 10 × (2 + 19 × 2) = 10 × 40 = 400