Using Normal Distributions to Estimate Populations Practice Test

The lifetimes of a certain brand of light bulb are approximately normal (based on quality-control data, the distribution is roughly bell-shaped). Lifetimes have mean $\mu = 1000$ hours and standard deviation $\sigma = 100$ hours. Approximately what percent of bulbs last between 900 and 1100 hours? (Use the empirical rule.)

A factory monitors the thickness of a coating, which is approximately normal (the histogram is roughly symmetric and bell-shaped). Thickness has mean $\mu = 1.20$ mm and standard deviation $\sigma = 0.05$ mm. Approximately what percent of items have thickness below 1.15 mm? (Use the empirical rule.)