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I think that donk on the turn means he has some piece of the board almost 100% of the time. Any piece of the board has you beat.

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How big of a part of his range is a J, and how often does he fold the J to a raise? We can pretty heavily discount a 5 from his range, right?

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Why would we discount a 5? He's in the BB, which means he could have damn near anything. His turn donk bet may simply represent straight forward play where he doesn't want to risk you checking behind on the turn.