Let P (t) represent the amount of a certain drug, in milligrams, that is in the bloodstream at time t hours. The rate at which the drug leaves the bloodstream is proportional to the amount of the drug in the bloodstream. The amount of the drug in the bloodstream can be modeled by dP/dt = kP, where k is a constant and t is time, in hours. At time t = 0,500 milligrams of the drug was administered. At the moment when the amount of the drug in the bloodstream is 50 milligrams, the amount is decreasing at a rate of 11.5 milligrams per hour. Required:
What is an expression for P (t)?