If first term is 1 , then find the sum of first 30 terms.

the sum of first 20 terms of an A.P.

with first term as a and common diff as d is

20/2[2a+19d]=20a+190d..(i0

and

 sum of the next 20 terms

=sum of the first 40 terms -sum of the first 20 terms

=40/2[2a+39d]-   20/2[2a+19d]

=20a+590d(ii)

acc to qn

20+190d=1/3[20a+590d]

solving we get

d=2a

is it given that first term is1?

then d=2

then you can find the sum of the first thirty terms using formula for sum of n terms.