There's a guy that goes to the gym training push ups. He goes there for 26 sessions. We don't know how many push ups he does on the first session. But we know that on every session he does the same reps he did on the previous session plus one. At the end of the 26 sessions he totalized 1235 push ups.

What's the initial number of push ups?

This is a case of an arithmetic progression, which is a simple problem to solve.