A mixing tank initially holds 100 L of brine containing 30 kg of salt. Brine with a concentration of 0.40 kg/L flows in at 3 L/min, while the well-mixed solution is pumped out at 5 L/min. Let S(t) denote the amount of salt (in kg) in the tank after t minutes, while the tank still contains liquid. Which differential equation correctly models S(t)?
a) dtdS=1.2−1005S
b) dtdS=1.2−100−2t3S
c) dtdS=1.2−100−2t5S
d) dtdS=100−2t5S−1.2