A tank is initially filled with 1000 litres of brine, containing 0.15 kg of salt per litre.

Question:

A tank is initially filled with 1000 litres of brine, containing 0.15 kg of salt per litre. Fresh brine containing 0.25 kg of salt per litre runs into the tank at a rate of 4 litres s^–1, and the mixture (kept uniform by vigorous stirring) runs out at the same rate. Show that if Q (in kg) is the amount of salt in the tank at time t (in s) then

dQ dt 10 250