At the end of the summer, I decided to drain the swimming pool (which has 1500 gallons of water in it). I noticed that it drains faster when there is more water in the pool. That was interesting to me, so I decided to measure the rate at which it drains. I found that it was draining at a rate of 1% every minute. How long will it take to have less than 500 gallons in the pool? (Justify your answer with a mathematical model of the problem situation: include an equation, table, AND graph)