Sometimes after a patient takes a drug, the amount of medication A in the bloodstream can be modeled by A = A₀e^-rt, where A₀ is the initial concentration in milligrams per liter, r is the hourly percentage decrease (in decimal form) of the drug in the bloodstream, and t is the elapsed time in hours. Suppose that a drug's concentration is initially 2 milligrams per liter and that r = 0.2. 

(a) Find the drug concentration after 3.5 hours. 

(b) When did the drug concentration reach 1.5 milligrams per liter?