5622. A sample from a large batch of electric lamps contains 140 lamps. The average lamp life of the sampled lamps was found to be 1100 hours. Find an approximate 0.994 confidence interval for the average lamp life of the entire lot if the standard deviation of the lamp life of the lot is known to be 39 hours.

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