A tank is initially filled with 1000 litres of brine containing 0.25 kg of salt/litre. Fresh brine containing 0.5 kg of salt / litre flows in at a rate of 3 litres per second and a uniform mixture flows out at the same rate. The quantity Q(t) kg of salt in the tank t seconds later is given by Q(t) = A + Be^–3t/1000 Find the values of A and B and sketch a graph of Q(t). Use the graph to estimate the time taken for Q(t) to achieve the value 375.