Correct the error as soon as you are confident it is real. A clean correction is better than building the rest of the recommendation on a number you know is wrong.

State what changed without over-apologizing: identify the arithmetic mistake, give the corrected figure, and explain whether it changes the implication.

If the recommendation flips, say so explicitly. For example: "With the corrected margin, option B is now more attractive, so I would revise my earlier recommendation."

Practice recovering in mocks rather than aiming only for perfect calculations. The ability to catch, correct, and update your conclusion is part of good analytical judgment. Use one concrete example to make the point easier to believe and easier to remember. Keep the answer anchored in the actual decision rather than trying to satisfy an imagined recruiter rule. For more detail, see the case interview math guide.

I would correct it immediately rather than hope the interviewer ignores it.