Question 1174421
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Hi
  Normal Distribution:  μ = 48m  and   σ = 7mos 
manufacturer does not want to exchange more than 10% of the watches
{{{z =blue (x - mu)/blue(sigma)}}}
  z = 1.282 appropriate to use.  invNorm(.90) = 1.282
  
 {{{1.282 = blue (x - 48)/blue(7)}}}
    x = 7(1.282) + 48 = 56.97 (rounding up to 57)  
 57mo the number of months the guarantee should be given for.)

Wish You the Best in your Studies.
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