GMAT Club Tests | Re: M15-03

Now I'm confused. Your initial solution said "...Notice that if we knew ANY term of the sequence we would be able to get all the other terms/numbers and thus answer the question."

Could you please explain how being able to determine all the other terms in the sequence from ANY term doesn't also enable one to determine the arithmetic mean, hypothetically speaking? Like what is the difference in sufficiency logic between both questions?Thanks