You walk up to a tank of water that can hold up to 20 gallons. When it is active, a drain empties water from the tank at a constant rate. When you first see the tank it contains 15 gallons of water. Three minutes later, that tank contains 10 gallons of water.

At what rate is the amount of water in the tank changing? Use a signed number, and include the unit of measurement in your answer.

How many minutes will it take for the tank to drain completely? Explain or show your reasoning.

How many minutes before you arrived was the water tank completely full? Explain or show your reasoning.