A tank holds 1000 gallons of water, which drains from the bottom of the tank in half an hour. The values in the table show the volume $V$ of water remaining in the tank (in gallons) after $t$ minutes.

$\displaystyle \begin{array}{|c|c|c|c|c|c|c|} \hline t \text{ (min)} & 5 & 10 & 15 & 20 & 25 & 30\\ V \text{ (gal)} & 694 & 444 & 250 & 111 & 28 & 0\\ \hline \end{array}$

If $P$ is the point (15,250) on the graph of $V$, find the slopes of the secant lines $PQ$ when $Q$ is the point on the graph with $t=$ (a) 5, (b) 10, (c) 20, (d) 25, and (e) 30. Round your answers to the nearest tenth.
(f) Estimate the slope of the tangent line at $P$ by averaging the slopes of two secant lines using the values of $t$ closest to $P$.

NOTE: It turns out that the slope of the tangent line is approximately -33.3 (Can you verify that?).

(a)
(b)
(c)
(d)
(e)
(f)