But acceleration is constant as given by the question:
s(t) = 1100-100t - 16t2
He could have given us a longer formula that contained things such as G, MEarth, REarth, but that would have been beyond the scope of this class. The formula, as he gives it implies a constant g.
Even so, if we were to use a non-constant g, then his method fails too.
I had a quick 10 seconds to ask him about this after class (many others were waiting to ask questions too), and he said I was marked wrong because he asked for an average, and by finding the first derivative, I was finding an instantanous velocity. His argument made no sense to me. On Monday I get a chance to spend more time to argue my point. So I appreciate you (and anyone else who cares to participate in this thread) arguing the teacher's side for me.
btw... I don't want to give anyone the impression that I dislike my teacher. He's one of the 5 best teachers I've had in my life, and a few other students agree. He explains things clearly, or I'd be visiting this sub-forum a lot more often.