Initially 10 grams of salt are dissolved into 20 liters of water. Brine with concentration of salt 3 grams per liter is added at a rate of 6 liters per minute. The tank is well mixed and drained at 6 liters per minute. Let xx be the amount of salt, in grams, in the solution after tt minutes have elapsed. Find a formula for the rate of change in the amount of salt, dx/dtdx/dt, in terms of the amount of salt in the solution xx.