Volume of a cylinder is given by πr^2h
Volume of the cylindrical water cooler is π x (12/2)^2 x 15 = 3.14 x 36 x 15 = 1,695.6 in^3
Volume of water in the cylindrical cooler is 1,695.6 / 2 = 847.8 in^2
Number of drinking cups that can be filled from the cooler is 847.8/20.14 = 42 cups.