[Brash620]

Look at it this way... if your drawing to a flush (with A,xs in your hand) an have three opponents you want to see one of nine cards. You're opponets could have six of them, but they could just as likely have none of them, there is no way to be 100% sure. If you assume that they all hold your flush card you are getting 41 to 3 odds on the flop if you assume that they have none of your flush cards you are geeting 41 to 9 on the flop. There are also other combinations that your opponets may or may not have. So to account for all the variations that could occur we factor in all unseen cards, the 9 flush cards and the 47 possible remaining cards.