Forum:Odds of pulling a card?

Well, I'm going to buy some old Duelist pack Yugi's, and I want to know the odds of me pulling a card. I know there are 5 cards to a pack, with 30 cards possible to get. I specifically want a mirror force, and I am not sure how many packs I should purchase to be almost guaranteed to pull one. I consider the odds are 1/30 of me getting it at each card, unless certain cards are more likely to be pulled. If so, then I am less likely to get it right? So I suppose my question is this. How many packs should I purchase to be very likely (at least a 90%) chance of getting at least one mirror force? I really thank any math wiz out there who can help me.