2) A shark tank filled with 100 gallons of fresh water is being simultaneously drained of the water inside and having salt water pumped in. The salt water being pumped in has a concentration of 500 grams of salt per gallon. Presume that the water in the tank is always of a uniform concentration of salt water at all times. If the salt water is pumped into the tank at a rate of 5 gallons a minute but the water in the tank is pumped out at only 4 gallons per minute (and presuming the tank is large enough that the water does not overflow), how long will it take for the concentration of the water in the tank to reach 400 grams of salt per gallon?