The sum of the first term and seventh term of an arithmetic progression is 6.If the twentieth term is 56,find the term whose value first exceeds 400.
a + (a + 6d) = 6 .
We are then given that the twentieth term is 56, so
a + 19d = 56 .
So now we have two equations in two unknowns, which we (you) can solve to find a and d. From there you should be able to complete the question.
Hope this helps.