A fish tank is filled with 10 gallons of dirty water, connected to a pump that filters at the rate of 1 gallon/hour. How long does it take until the tank is completely clean?

The last answer is true, but it is also useless in terms of an engineering project. A better answer to give is: do a few iterations of the decaying exponential to demonstrate your grasp on the problem. Then assign an good enough definition of completely clean and plug in numbers until you get around that number.