A tank is initially filled with 100 litres of salt solution containing 0.1 kg of salt per litre. Fresh brine containing 0.2 kg of salt per litre runs into the tank at the rate of 5 litres / minute and the mixture, assumed to be kept uniform by stirring, runs out at the same rate. Find the amount of salt at any time and determine how long will it take for this amount to reach 15 kg.