A pond contains L of pure water and an uknown amount of an undesirable chemical. Water contaninig kg of this chemical per liter flows into the pond at a rate of L/h. The mixture flows out at the same rate, so the amount of water in the pond remains constant. Assume that the chemical is uniformly distributed throughout the pond. Let be the amount of chemical (in ) in the pond at time hours. (a) Write a differential equation for the amount of chemical in the pond