Help with stats!! 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 46.6 months and a standard deviation of 6.9 months. (a) 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 (to the nearest month)?

Question

e.mccormick · Answer

If you think about it, you want all except 15% should last this long.  So we have a normal curve like this:|dw:1368758006092:dw|What it is asking is where that line is.

So what Z score would result in 15% being to the right of that bar?  Once you know that, working the 6.9 months into the answer should not be too hard.