Answer
$12$ miles per hour
Not realistic
Work Step by Step
The function modeling the driving time is:
$$T(x)=\dfrac{470}{x+70}+\dfrac{250}{x+65}.$$
On the graph of the function we draw the line $y=9$ and find the intersection point with the function's graph:
$$(11.8,9).$$
So about $12$ miles per hour over the speed limits would be necessary in order to drive for $9$ hours. This is quite a substantial amount over the speed limits which might be difficult and dangerous to accomplish (due to safety and fines).
