Distrubution of the sample proportion problem help?

According to the National Survey of Salaries and Wages in Public Schools, the mean salary paid to public high school principals in 2004-2005 was $71,401. Assume the standard deviation is $26,145. What is the probability that a random sample of 100 public high school principals has an average salary under $65,000? Justify and explain your reasoning.

I haven't done any like this in class, I only did the problems with confidence interval's in them. I looked through my textbook and I do not see any example problems.