The sum of first 20 odd natural numbers is
Here A.P is 1,3, 5, ……
a= 1, d= 2 and n = 20
Sn = (2a + (n–1) d)
Sn = 20/2 (2 x 1 + (19) 2)
Sn = 10 (2 + 38)
Sn= 400