document.write( "Question 442736: The mean lifetime of a wristwatch is 25 months, with a standard deviation of 5 months. If the distribution is normal, for how many months should a guarantee be made if the manufacturer does not want to exchange more than 10% of the watches? Assume the variable is normally distributed. \n" ); document.write( "
Algebra.Com's Answer #305425 by stanbon(75887)\"\" \"About 
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The mean lifetime of a wristwatch is 25 months, with a standard deviation of 5 months. If the distribution is normal, for how many months should a guarantee be made if the manufacturer does not want to exchange more than 10% of the watches? Assume the variable is normally distributed.
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\n" ); document.write( "Find the z-value with a left tail of 10%.
\n" ); document.write( "invNorm(0.1) = -1.2816
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\n" ); document.write( "Find the corresponding x-value:
\n" ); document.write( "x = zs+u
\n" ); document.write( "x = -1.2816*5+25
\n" ); document.write( "x = 18.59 months
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\n" ); document.write( "Guarantee them for 18.59 months. Only
\n" ); document.write( "10% will have failed by that time.
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\n" ); document.write( "Cheers,
\n" ); document.write( "Stan H.\r
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