You have a sample of test scores with a mean of 78.2 and a standard deviation of 9.0. To use a goodness of fit test to determine if a sample of scores comes from a normal population, you divide the normal distribution into 10 equal-probability intervals. What score is at the boundary between the upper 20% and the next lower interval?

I am confused how to get started on this problem because the book I am using does not give any examples in which they give a mean and a standard deviation. Any help on how to solve this problem would be appreciated.