Answer
See below.
Work Step by Step
Obviously it is worth having at least $50$ trees per acre. Then if we increase the number of trees by $x$, the function modeling the income will be: $(50+x)(320-4x)=-4x^2+120x+16000$
The maximum value is at $x=-\frac{b}{2a}=-\frac{120}{2\cdot(-4)}=15.$ Hence the maximum value is $f(15)=-4\cdot15^2+120(15)+16000=16900.$
Thus the number of trees should be $50+15=65$ and the income will be $16900$.
