Answer
See graph.
Work Step by Step
Step 1. It takes $\frac{100}{5}=20\ min$ to fill the tank, thus during $0\le x \le 20$, we have $f(x)=5x$ where $x$ is the time in minutes.
Step 2. It takes $\frac{100}{2}=50\ min$ to empty the tank, thus during $20\lt x \le 70$, we have $f(x)=100-2(x-20)x$ where $x$ is the time in minutes.
Step 3. Combine the above results, we have $f(x)=\begin{cases} 5x,\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 0\le x \le 20 \\ 100-2(x-20)x,\ 20\lt x \le 70 \end{cases}$
Step 4. We can graph the above function as shown in the figure.
