Answer
They should decrease the price by $20$.
Work Step by Step
$R(x)=(320-20x)(70+5x)=-100x^2+200x+22400$
Let's compare $R(x)=-100x^2+200x+22400$ to $f(x)=ax^2+bx+c$. We can see that a=-100, b=200, c=22400. $a\lt0$, hence the graph opens down, and its vertex is a maximum. The maximum value is at $x=-\frac{b}{2a}=-\frac{200}{2\cdot(-100)}=1.$ Hence the maximum value is $f(1)=-100(1)^2+200(1)+22400=22500.$
Thus they should decrease the price by $20$.
