Question

The amount of time a certain brand of light bulb lasts is normally distributed with a mean of 1200 hours and a standard deviation of 50 hours. What is the probability that a randomly chosen light bulb will last more than 1280 hours, to the nearest thousandth?

Answer 1

Z = (1280 − 1200)/50 = 80/50 = 1.6.
P(X > 1280) = 1 − Φ(1.6) = 1 − 0.9452 = 0.0548 ≈ 0.055 (to the nearest thousandth).