18. A water tank has the shape obtained by revolving the parabola ??2 = ???? around they -axis. The water depth is 4 ft at 12 noon, when a circular plug in the bottom of the tank isremoved. At 1 P. M....

18. A water tank has the shape obtained by revolving the parabola ??2 = ???? around they -axis. The water depth is 4 ft at 12 noon, when a circular plug in the bottom of the tank isremoved. At 1 P. M. the depth of the water is 1 ft.(a) Find the depth y (t) of water remaining after t hours.(b) When will the tank be empty?(c) If the initial radius of the top surface of the water is 2 ft., what is the radius of the circularhole in the bottom?