Question

The amount of time a certain brand of light bulb lasts is normally distributed with a mean of 1700 hours and a standard deviation of 90 hours. What percentage light bulbs last less than 1550 hours, to the nearest tenth?

Answer 1

z = (1550 − 1700)/90 = −150/90 = −1.6667.

P(X < 1550) = Φ(−1.6667) ≈ 0.0478 = 4.8% (to the nearest tenth of a percent).