(1 point) Initially 10 grams of salt are dissolved into 40 liters of water. Brine with concentration of salt 4 grams per liter is added at a rate of 2 liters per minute. The tank is well mixed and drained at 2 liters per minute. a. Let x be the amount of salt, in grams, in the solution after 7 minutes have elapsed. Find a formula for the incremental change in the amount of salt. Ax, in terms of the amount of salt in the solution x and the incremental change in time At. Enter At as Deltat. Ax = grams b. Find a formula for the amount of salt, in grams, after 1 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