50 gallons fluid + (4gallon Soln/min - 5 gallons of fluid)*t
Remember, 4 gallons are being pumped IN every time 5 gallons is removed. So the difference is -1. So the amount of fluid in the tank at any time t is given by the function 50-t.
Remember that's gallons of FLUID = water +alcohol. So to get the rate out for alcohol alone you just have to pay attention to the units.
has the units: gallons of Alcohol / min.