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.