5621. A sample from a large batch of electric lamps contains 110 lamps. The average burn time of the sampled lamps was found to be 1600 hours. Find an approximate 0.994-confidence interval for the average lamp burn time of the entire lot if the standard deviation of the lamp burn time of the lot is known to be 21 hours.

Detailed solution. Decorated in Microsoft Word 2003 (Quest decided to use the formula editor)