If you would like to know how many cubic feet of water are needed to completely fill the tank, you can calculate this using the following steps:
a = 2 1/2 feet * 2 1/2 feet * 2 1/2 feet = 2 1/2 * 2 1/2 * 2 1/2 = 5/2 * 5/2 * 5/2 = 125/8 = 15 5/8 cubic feet
The correct result would be 15 5/8 cubic feet.