Find the sum of 21 terms of an AP whose middle term is 20

middle term = 21+1/2 = 11
11th term = 20=a+10d
S of 21 terms = 21/2(2a+20d)
= 21/2{2(a+10d)}
= 21/2 (2*20)
=21/2(40)
=21*20
= 420