I don't think you're using the word equity properly, but I don't think that's what you're asking either.
I think you want to know the
probability of making the nut low (a) on the turn and (b) by the river.
(a) is easy to calculate and you've basically done it. There are 45 unseen cards; of those, 21 will give you the nut low on the turn. 21/45 = 46.7%.
(b) is a little bit more complicated. To figure the probability of making a low (not necessarily the nut low but any low) by the river, we have to figure the probability of
not making a low and then subtract that from 100%.
On the turn there are 24/45 unseen high or pairing cards (when I say pairing cards here, I mean the remaining three 7's and the remaining three 8's). After one of those high or pairing cards comes on the turn, there are 23/44 unseen high or pairing cards on the river.
To calculate the probability of a high or pairing card coming on
both the turn and the river, we multiply these two probabilities:
24/45 x 23/44 = 53.3% x 52.3% = 27.9%
That's the probability of
not making a low by the river. So the probability of making a low by the river is that probability subtracted from 100%:
100% – 27.9% = 72.1%.
However, sometimes you will get double-counterfeited, and your low will
not be the nut low at showdown. In order to get double-counterfeited, 9/45 unseen pairing cards (when I say pairing cards here, I mean the nine low cards that pair either the A, 2, or 3 in your hand) have to come on the turn, and 6/44 unseen pairing cards have to come on the river. Here's the probability of that happening:
9/45 x 6/44 = 20% x 13.6% = 2.7%
We then take the probability of making any low by the river and subtract the probability of making a non-nut low to figure the probability of making the nut low by the river:
72.1% – 2.7% = 69.4%
If you want to express this as odds, you convert it to a ratio as follows:
100% – 69.4% = 30.6%
69.4 to 30.6 = 2.27 to 1
(This article explains the difference between probability and odds, in case that's part of your question:
http://senseaboutscienceusa.org/know...d-probability/.)
To calculate your equity in the hand, we'd need to know how many opponents you have and what their exact cards are (including suits) so we could calculate each player's probability of making the winning high hand and/or the winning low hand.
For example, you could make the nut low but win only one-quarter or one-sixth of the pot—that would reduce your equity considerably. Conversely, you could make the nut low and also make the nut flush—that would increase your equity considerably.
Last edited by agamblerthen; 08-21-2017 at 03:37 AM.
Reason: Had to correct my math errors.