An exponential random variable X has been used to model the relief times (in minutes) of arthritic patients who have taken an analgesic for their pain. Suppose that the density function for X is f (x) = ke^-kx and that a certain analgesic provides relief within 4 minutes for 75% of a large group of patients. Then, we may estimate that Pr (X ≤ 4) = .75. Use this estimate to find an approximate value for k.