A Mixture Problem A tank initially contains 10 gal of brine with 2 lb of salt. Brine with 1.5 lb of salt per gallon enters the tank at the rate of 3 gal/min, and the well-stirred mixture leaves the tank at the rate of 4 gal/min. It can be shown that the amount of salt in the tank after $t$ min is $x$ lb, where
$$x=f(t)=1.5(10-t)-0.0013(10-t)^{4} \quad(0 \leq t \leq 10)$$
What is the maximum amount of salt present in the tank at any time?