Question

The sum of first five terms of an AP and the sum of the first seven terms of the same AP is 167. If the sum of first ten terms of this AP is 235, find the sum of first twenty terms.

Answer 1

S5+S7=167

S10=235

i think your doubt is cleared if not see below complete answer

a+ 4d+a+6d=167. eq1

a+9d=235 or 2a+18d=470. eq2

sub eq1 and eq2

we get

8d= 303 further calculate yourself