What is it with these anachronistic instructors who insist on doing these things by hand?. Tell your instructor I said to look at the calendar. It's 2008, not 1908. Does he/she want you to calculate square roots by hand as well?.
Anyway, you can google and find some things. Here is what wiki has to say about it.
Percentile - Wikipedia, the free encyclopedia
There are different formulas and methods. Excel may use a different program compared to another. That is why you may get different results from different applications.
Here is something I had on hand: