A tank contains 100 L of pure water. Brine that contains 0.1 kg of salt per liter enters the tank at a rate of
10(L)/(m)in
. The solution is kept thoroughly mixed and drains from the tank at the same rate. How much salt is in the tank after 6 minutes? One model for the spread of an epidemic is that the rate of spread is jointly proportional to the number of infected people