A tank initially holds 100 liters of a brine solution containing 0.25 kg of salt. At t=0 another brine solution containing 0.25 kg of salt per liter is poured into the tank at a rate of 3 liters/min, while the well-stirred mixture leaves the tank at the same rate.

a, Find the amount of salt in the tank at any time t.

b. Find the time at which the mixture in the tank contains 0.5 kg of salt.