(1 point) Initially 15 grams of salt are dissolved into 20 liters of water. Brine with concentration of salt 4 grams per liter is added at a rate of 3 liters per minute. The tank is well mixed and drained at 3 liters per minute. a. Let x be the amount of salt, in grams, in the solution after t minutes have elapsed. Find a formula for the rate of change in the amount of salt, dx/dt, in terms of the amount of salt in the solution x. dx dt grams/minute b. Find a formula for the amount of salt, in grams, after t minutes have elapsed. x(t) = grams c. How long must the process continue until there are exactly 20 grams of salt in the tank? minutes