Chapter 2.3, Problem 3P

A tank originally contains $160$ gal of fresh water. Then water containing $\frac{1}{4}lb$ of salt per gallon is poured into the tank at a rate of $4gal/\min$, and mixture is allowed to leave at the same rate. After $8$ min the process is stopped, and fresh water is poured into the tank at a rate of $6gal/\min$, with the mixture again leaving at the same rate. Find the amount of salt in the tank at the end of an additional $8$ min.