2017 Cayley Problem 5

The graph shows the volume of water in a 300 L tank as it is being drained at a constant rate. At what rate is the water leaving the tank, in litres per hour?

\textbf{(A)}\ 12\quad \textbf{(B)}\ 20\quad \textbf{(C)}\ 2.5\quad \textbf{(D)}\ 5\quad \textbf{(E)}\ 15

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Cayley contests here.