Probability
posted by ryan .
You are taking a multiple choice test that has 20 questions with 4 possible answers each. You randoming quess each answer. What is the probality of getting 7 questions correct?

Pr(7 right, 13 wrong)= .25^7 * .75^13 * number of ways
the number of ways is 20!/(7!13!)
((20 !) / ((7 !) * (13 !))) * (.25^7) * (.75^13) = 0.112406195
check my thinking. 
And here are some probability tutorials you might find handy:
http://search.yahoo.com/search?fr=mcafee&p=probability+tutorial
Sra 
Bonston high school