PRACTICE ANOTHER Many physical processes are exponential in nature. A typical way of cleaning a tank of contaminated water is to run in clean water at a constant rate, stir, and let the mixture run out at the same rate. Suppose there are initially 100 pounds of a contaminant in a large tank of water. Assume that the cleaning method described above removes 10% of the remaining contaminant each hour. Find an exponential formula that gives the number of pounds of contaminant left in the tank after t hours.