Instruction 7.3 The Lifetimes of a Certain Brand of Light Bulbs Are

Essay

Instruction 7.3
The lifetimes of a certain brand of light bulbs are known to be normally distributed with a mean of 1,600 hours and a standard deviation of 400 hours. A random sample of 64 of these light bulbs is taken.
-Referring to Instruction 7.3,the probability is 0.20 that the sample mean lifetime differs from the population mean lifetime by at least how many hours?