statistics
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 zscore using a ztable. Remember the question is asking that the dealer wants only 5% of the cars to fail. Keep that in mind when looking for the zscore.
z = (x  mean)/sd >zscore formula
Substitute the zscore you found, the mean, and the standard deviation into the formula, then solve for x.
I'll let you take it from there. 
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