Correct it politely if the difference could affect the analysis. It is better to clarify the number than let the case continue on a false premise.

Case interviews are collaborative enough that small clarifications are normal.

You can say, 'Just to make sure we are aligned, I had calculated 12%, not 20%.'

Clarify immediately when the number matters, keep the tone neutral, and continue. If useful, this guide on first-round consulting interviews gives more detail.