A tank originally contains 100 gallons of fresh water. Then, water containing 0.5 lb of salt per gallon is poured into the tank at a rate of 2 gallons per minute, and the mixture is allowed to leave at the same rate. After 10 minutes, the process is stopped and fresh water is poured in the tank at a rate of 9 gallons per minute, with the mixture again leaving at the same rate. Find the amount of salt in the tank at the end of an additional 10 minutes.