You mean that the low will come on the river given two low cards already on the board?
There are 8 low cards in each suit with 4 suites, so 32 low cards in the deck. You've see 2 of them, so there are 30 left. However, you can't pair one of the existing low cards on board, so you have to remove those 6 from the pool of cards that help you. Therefore we subtract 6 cards to get 24 cards that will make a low.
a) Assumes you haven't got a hand.
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52 total cards total
4 cards seen on board
6 cards won't help
42 cards unseen and possibly helpful.
24 will help.
42:24 = 1.75:1 against the low getting there on the river.
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b) You hold 0 low cards.
<font class="small">Code:</font><hr /><pre>
52 total cards total
4 cards seen on board
4 cards in your hand
6 cards won't help
38 cards unseen and possibly helpful.
24 will help.
38:24 = 1.6:1 against the low getting there on the river.
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The rest just progress from there. I think the math is correct, but I'm sure someone will correct if it's not.
Regards,
T