OR A tank originally contains 100 gallon of fresh water. Then water
containing 0 5 Lb of salt per gallon is pourd into the tank at a rate of 2
gal/minute, and the mixture is allowed to leave at the same rate. After 10
minute the process is stopped, and fresh water is poured into the tank at a
rate of 2 gal/min, with the mixture again leaving at the same rate. Find the
amount of salt in the tank at end of an additional 10 minutes.​