You are the owner of an auto repair service. History tells you it takes on average 45 minutes to complete a repair job. You have determined the standard deviation for a job is 6 minutes. A women comes into your shop and tells you she must leave her car for repair, and that she will be shopping at the mall across the street. She says she will be back no earlier than 38 minutes, but she absolutely must leave no later than 55 minutes to pick up her child from school. What is the probability her car will be repaired during the 38 to 55 minute timeframe?
Z = (x-μ)/SD, where x = score, μ = mean, and SD = standard deviation.
In table on back of your statistics book called something like "areas under normal distribution," use Z scores to find areas between scores and mean. Add the two values.