Calculating Odds with Two Cards to Come

[ne14dirt]

Disclaimer: I’m bad at math so please bear with me. I’d also like to say that I am sure this topic has been presented before but I did a search and could not find it. When I search the internet I kept coming up with odd’s being calculated on the turn as opposed to on the flop. Now on to the question.

Please explain the math behind calculating odds with two cards to come. I’ve been using the rule of four and two as presented by Phil Gordon in his “Little Green Book” which works with “good enough” accuracy but as of a couple of hours ago it started bothering me that I can get my head around the percentages with one card to come but I can not for the life of me figure the math with two cards to come.

Example as presented in “Little Green Book”

I have 10c, 9d
My opponent has As, Kd
Flop Ac, 10d, 7s

Using the rule of four I take 5 as my number of outs times 4 = 20 or 20% that my card will hit.

But if I try and think it out, I tell myself I have 5 outs and for the discussions sake I am sure my opponent has A,K which leaves 45 cards left in the deck., five of which are my outs so I am 40/5 or 8/1. I’m guessing I’m 8 to1 to hit my card on the turn…..right. But 8-1 is only 12.5%......how do I calculate to include the river card.