A tank contains 200 gallons of water in which 300 grams of salt is dissolved. A brine solution containing 0.4 kilograms of salt per gallon of water is pumped into the tank at the rate of 5 liters per minute, and the well-stirred mixture is pumped out at the same rate. Let A(t) represent the amount of salt in the tank at time t. The amount of salt in the tank at time t is:
a. A(t) = 80 - 79.7e-t/40
b. A(t) = -80 + 80.3et/25
c. A(t) = 200 - 199.7e-t/40
d. A(t) = -200 + 200.3et/40
e. A(t) = 200 - 100e-t/40