Differential Equations: A differential equation is an equation that involves and unknown function and some of its derivatives. Differential Equations are a large subject on which we teach several courses. In Calculus you only get a first exposure to them and you see some very simple examples. Set up and solve a differential equation to solve the following problem: You have a pond with decorative fish in your back yard. The pond holds 800 gallons of water. Once a week you pour fresh water into the pond at the rate of 100 gallons per hour. The pond is filled to the brim, and so as you pour water into the tank water flows out at the same rate. There is a pump in the pond that keeps the water perfectly mixed. Your goal is to add water until any pollutants in the pond are reduced by a factor 1/2. You keep the fresh water flowing for hours. (Remember that ww expects your answer to be accurate within one tenth of one percent. You may want to enter a mathematical expression rather than a numerical value.)