The tank is filled with $100$ L of substance A at a concentration of $10$ g$/$L$.$ A at a concentration of $1$ g$/$L is fed into the tank at a flow rate of $5$ L$/$min$.$ There is an outlet flow at the outlet of the tank at a flow rate of $5$ L$/$min and the tank is continuously stirred. Substance A is converted into substance B in the tank. $($Reaction rate constant: k$=0\mathrm{.}1$ L$/$mol$.$ min$).$ What is the equation relating the concentration of substance B in the tank to time?