Question 1003374
nth. term of an AP is a+(n-1) d

a8 = a+7d
a4 = a+3d

2(a+3d) = a+7d

2a+6d = a+7d

a=d

a20 = a+19d

40 = a+19a

20a=40
a= 2
d=2

Sn = {{{(n/2)(2a+(n-1)d)}}}

S20 = {{{(20/2)*(2*2+19*2)}}}

= 10(4+38)
10*42
=420

S20 = 420