The 3rd term of an A.p is more than the first term, while the fifth termis 15 more than the second term.find the sum of the 8 the and 15th term of the A.p if to 7th term is 7 times

The question was not complete, either time... You say

The 3rd term of an AP is more than the first term

okay -- how much more?

all we really know is that d=5.

and don't even get me started on

if to 7th term is 7 times

what does that even mean?

get a grip, willya?