**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?