Question: A 1 liter tank is filled with a salt solution containing 4 grams of dissolved salt. At t = 0, a hole is drilled in the bottom of the tank and the solution drains out at 13 liters per minute. At the same time, fresh water is being pumped in at 13 liters per minute. Let x(t) be the amount of salt in grams after t minutes.

a) Find a formula for x(t) by solving the appropriate differential equation.

b) How long will it take until 1 gram of salt is left in the tank?