Question

The amount of time a certain brand of light bulb lasts is normally distributed with a mean of 1100 hours and a standard deviation of 20 hours. Using the empirical rule, what percentage of light bulbs last between 1060 hours and 1140 hours?

Answer 1

1060 and 1140 are 40 hours from the mean, i.e. ±2 standard deviations (40/20 = 2). By the empirical rule about 95% of values lie within ±2σ, so approximately 95% of the bulbs last between 1060 and 1140 hours.