Hi there maggsmaggs,
I think you are trying to solve
After some further calculations you should get
1) A tire company has developed a new type of steel-belted radial tire. Extensive testing indicates the population of mileages obtained by all tires of this new type is normally distributed with a mean of 40,000 miles and a standard deviation of 4,000 miles. The company wishes to offer a guarantee providing a discount on a new set of tires if the original tires purchased qualify the guarantee mileage. What should the guaranteed mileage be if the tire company desires that no more than 2 percent of the tires will fail to meet the guaranteed mileage?
-Do you go to the left part of the curve to answer this question, thus a negative z-score? Or is the z-score positive?
-If you do go to the left, what Z-score should you use? If you use the closest to .5-.02, you use -2.05, but that corresponds to .4798, which would be surpassing that 2% limit. Using -2.06 would thus seem to make more sense as a Z-score even though it's further from the .48 than -2.05. That make sense?
Thanks in advance!