A student learns a certain amount of material for some class. Let f (t) denote the percentage of the material that the student can recall t weeks later. The psychologist Hermann Ebbinghaus found that this percentage of retention can be modeled by a function of the form f (t) = (100 - a)e^-λt + a, where l and a are positive constants and 0 < a < 100. Sketch the graph of the function f(t) = 85e^-0.5t + 15, t ≥ 0.