<PRE><B>Question 1074392</B>
Quality-control studies for Speedy Jet computer printers show that the lifetime of the printer follows a normal distribution with mean 4 years and standard deviation 0.78 years. The company will replace any printer that fails during the guarantee period. How long should Speedy Jet printers be guaranteed if the company wishes to replace no more than 10% of the printers? 
Find the z-value with a left tail of 10%
Z=invNorm(0.10) = -1.2816
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Comment:: The time value you want is 1.2816 standard deviations
below the mean

X = -1.2816*0.78 + 4 = 3 years
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Cheers,
Stan H.
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