Need help calculating the slope when t=5. "The missing areas"

A tank holds 2000 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. t (min) 5 10 15 20 25 30 V (gal) 1398 866 480 210 52 0 Exercise (a) If P is the point (15, 480) on the graph of V, find the slopes of the secant lines PQ when Q is the point on the graph with the following values. Q (5, 1398) (10, 866) (20, 210) (25, 52) (30, 0) Step 1 Slopes of secant lines passing through P(15, 480) and Q(t, V(t)) can be calculated using msec V(t) - 480 t - 15 We begin by calculating the slope when t = 5. msec = V(5) - 480 5 - 15 - 480 = (rounded to the nearest tenth) Submit Skip