Reference for Makkot 44:9

אמר רב ששת לא קשיא הא דאמדוהו לארבעים וחדא הא דאמדוהו לארבעים ותרתי:

Rav Sheises said, it is not a contradiction. Here is where they evaluated him to be able to withstand forty-one lashes [which would be rounded down to thirty-nine - only one set]. Here is where they evaluated him to be able to withstand forty-two lashes [which would include three of the second set as well].

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