A plugged sink is filled with water. After the plug is removed, water drains at a constant rate. The amount of water remaining in a sink is given by the equation \(A=15-5x\), where \(x\) is the number of minutes after the plug is removed and \(A\) is the volume, in gallons, of water remaining in the sink. In the equation, what are the meanings of the numbers \(15\) and \(5\)?