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 $4 \mathrm{gal} / \mathrm{min}$. It can be shown that the amount of salt in the tank after $t$ min is $x$ lb, where
What is the maximum amount of salt present in the tank at any time?