1 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 t min V gal 5 694 444 250 111 10 15 20 25 28 30 0 a 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 5 10 20 25 and 30 b Estimate the slope of the tangent line at P by averaging the slopes of two secant lines c Use a graph of V to estimate the slope of the tangent line at P This slope represents the rate at which the water is flowing from the tank after 15 minutes