1. The problem statement, all variables and given/known data 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 the rate of 4 litres per second, and the mixture (kept uniform by stirring) runs out at the same rate. Show that if Q (in kg) is the amount of salt in the tank at the time t (in seconds) then d/dt Q(t) = 1 - ((Q(t))/(250)) 2. Relevant equations I know I have to take the derivative since this is an optimization problem and it includes rates, but nothing else. 3. The attempt at a solution I know that the there are 1000 litres of brine and 0.15 kg of salt per litre. the 0.15 kg is already in. In addition, 0.25 kg of salt per litre runs into the tank at a rate of 4 litres per second. This is where I'm having trouble. How do I formulate an equation in relation to the one given? Please help.