[ccwhoelse?]

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How do you quickly calculate these sort of odds when playing?

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you don't. you should know some of the more common odds off hand and just know that long shot draws go a long way as far as adding to your chance of improving your hand to the best hand. so if a backdoor flush or an inside straight draw is your only chance to win, then you might not want to continue, but coupled with other outs it turns a fold into a call.