Question

The amount of time a certain brand of light bulb lasts is normally distributed with a mean of 1900 hours and a standard deviation of 20 hours. Using the empirical rule, what percentage of light bulbs last between 1840 hours and 1960 hours?

Answer 1

1840 and 1960 are 1900 ± 60 = 1900 ± 3(20), i.e. ±3σ. By the empirical rule about 99.7% of values lie within 3 standard deviations, so ≈99.7% of bulbs.