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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What is z-score; and where did the 0.5 come from?
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