In chemistry class Andrew has earned scores of 64 69 and 73 on three tests he must maintain an average of 72

You haven’t told me what the question is.   But I put the mouse
to my forehead, closed my eyes, took a deep breath, and I could
see it shimmering in my mind’s eye.   It was quite fuzzy, but I think
the question is
                   "What score does Andrew need on the next test
                     in order to raise his average to 72%?"
The whole experience drew an incredible amount of energy
out of me, and the mouse is a total wreck.   So we’ll just go ahead
and answer that one.   I hope it’s the correct question.
The average score on 4 tests is
                                     (1/4) (the sum of all the scores).
In order for Andrew to have a 72% average on 4 tests,
the sum of the 4 scores must be
                                         (4) x (72%)  =  288%.
Out of that total that he needs, he already has 
                            (64% + 69% + 73%) = 206%
on the first three tests.
So in order to average 72% for all 4 tests,
he’ll need to score
                             (288%  -  206%)  =   82%
on the fourth one.