A tank has 500 litres of salt water, which includes 10 kg of salt. At time t = 0, a solution of salt water (concentration 0.01 kg/l) is pumped in the tank at a rate of 10 l/min. At the same time, the tank starts to be emptied at rate of 20 l/min.

Use your knowledge of differential equations to solve an equation of the amount (mass) of salt within the tank at any given time t.