When a pharmaceutical drug is injected into the bloodstream, its concentration at time t can be approximated by C(t) = Coekt, where Co is the concentration at t = 0. Suppose the drug is ineffective below a concentration C and harmful above a concentration C2. Then it can be shown that the drug should be given at intervals of time T, where C2 1 A certain drug is harmful at a concentration five times the concentration below which it is ineffective. At noon an injection of the drug results in a concentration of 2 mg per liter of blood. Five hours later the concentration is down to 1 mg per liter. How often should the drug be given?