posted by victoria .
A used car dealership has found that the length of time before a major repair is required on the cars it sells is normally distributed with a mean of 10 months and a standard deviation of 3 months. If the dealer wants only 5 percent of the cars to fail before the end of the guarantee period for how many months should the cars be guaranteed?
You will need to find the z-score using a z-table. Remember the question is asking that the dealer wants only 5% of the cars to fail. Keep that in mind when looking for the z-score.
z = (x - mean)/sd -->z-score formula
Substitute the z-score you found, the mean, and the standard deviation into the formula, then solve for x.
I'll let you take it from there.