Question 627275
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Hi, Re TY, Yes at less than 26 months... 50% would would be defective... 
average cell phone will be <u>26 months before repairs are needed</u>. If the standard deviation for the data is 6 months
 z = (x - 26)/6
what length of time{{{highlight(x)}}} should be set for a warranty such that less than 10% of all phones will need repairs during the warranty period.
 {{{highlight(-1.28)}}} = (x-26)/6      | Note: Excel NORMSINV(0.1)= -1.2816
{{{6*-1.2816 +26}}} = x 
{{{drawing(400,200,-5,5,-.5,1.5, graph(400,200,-5,5,-.5,1.5, 0,exp(-x^2/2)), blue(line( -1.2816,0, -1.2816,exp(-1.2816^2/2))),locate(4.8,-.01,z),locate(4.8,.2,z))}}}