A tank holds 2,000 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)	1,398	880	510	212	58	0

(a)If P is the point (15, 510) on the graph of V, find the slopes of the secant lines PQ when Q is the point on the graph with

t = 5,

10, 20, 25, and 30.(b)Estimate the slope of the tangent line at P by averaging the slopes of the two adjacent secant lines corresponding to the two points closest to P.