A simple mixing setup is shown below. At time $t=0,$ the inlet and outlet valves are opened, letting brine into and out of the tank. The concentration of the inlet brine is $0\mathrm{.}5lb$ NaCl$/$gal$,$ and the inlet flow rate is $6$ gal$/$min$.$ The outlet flow rate is $4$ gal$/$min and the initial brine concentration in the tank is $10$ lb NaCl in $100$ gal of water. How much NaCl is in the tank after $30$ min $?$