thats smart, man ill never be able to do that
I am not good at this kind of sums
Try this developmental plan:
Take about 25 questions that are half probability and the other half Perms & Combs of no less than Medium difficulty
Do them all. Doesn't matter what number you get right or wrong. Once done understand the way the problems are meant to be solved.
Do them all again the next day. Also, goto gmatclub.com > Courses > Quantitive and understand the tutorials there.
Now you will be all set to surprise yourself, except for one more thing -
Do about 4-5 questions from the file at this link everyday. Its important to keep in touch.
Take out about an hours or 2 hours time every week (weekend's work for me) to revise the stuff you did on Days 1 & 2.
If you've been crazy enough to do the above, my guess is you'll pray for Perms & Combs, and Probability to come in your GMAT.