SOLUTION: Quick Start Company makes 12-volt car batteries. After many years of product testing, the company knows that the average life of a Quick Start battery is normally distributed, with

Question 378308: Quick Start Company makes 12-volt car batteries. After many years of product testing, the company knows that the average life of a Quick Start battery is normally distributed, with a mean of 44.0 months and a standard deviation of 8.3 months.
(a) If Quick Start guarantees a full refund on any battery that fails within the 36-month period after purchase, what percentage of its batteries will the company expect to replace?

(b) If Quick Start does not want to make refunds for more than 15% of its batteries under the full-refund guarantee policy, for how long should the company guarantee the batteries?

Answer by ewatrrr(24785) (Show Source): You can put this solution on YOUR website!



Hi

*Note: 

 = -8/8.3 = -.9639 

P(z <-.9639) = .1675  Expect to replace 16.75%



P(z = -1.0364) = .15

-1.0364 = (x - 44)/8.3

8.3*(-1.0364) + 44 = x

-8.6021 + 44 = 35 months



 

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