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Normal Distribution problem

The length of time that light bulbs will be functional is normally distributed with a mean of 800 hours and a standard deviation of 160 hours. If testing determines that 0.3% of the bulbs fail within the first x hours then x in hours is:

The corresponding z -score for 0.3% or 0.003
0.5 -0.003= 0.497
z-score will be -2.75
(x-800)/ 160 = -2.75

x= 360 hrs.

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