An important thing to practice is turning a real world problem into a mathematical model. Let's practice this with a brine mixing example. Atank holds 2500 litres of water in which 2000 grams of salt has been dissolved. Brine which contains 5 grams of salt per litre is mm in at a rate of 25 litres per minute. [See below for a picture of the tank). The solution is thorougth mixed and is extracted from the tank at the same rate. Let a: be the number of grams of salt in the tank after: minutes. Based on the information given . Initially there are grams of salt in the tank. . salt is being added to the tank at a rate of 125 9 grams per minute . the number of litres in the tank at time t is 2500 9 . The salt is evenly distributed in the tank by mixing. Hence if there are a: grams of salt in the tank, then one litre taken a: from this tank contains n I grams of salt. and 25 litres contains T00 grams of salt. The change in salt with respect to time is described by the differential equation do: , a: E = Inow outow 125 - m