The arithmetic itself does not necessarily become dramatically harder in a final round. What usually changes is the standard around how confidently and cleanly you handle the quantitative part while also keeping the broader case moving.

A senior interviewer may challenge an assumption, ask you to interpret the result immediately, or push you to estimate something without much setup. That can make the math feel harder even when the underlying calculation is fairly ordinary.

For prep, I would focus on reliable execution: set up the equation clearly, keep units visible, talk through the logic, and sanity-check the answer before you move on. You want the interviewer to trust the process even if the numbers are not pretty.

Also practice translating the number into a business implication right away. Final-round math is rarely just about landing on the answer; it is about showing that you know why the answer matters. A useful reference is case interview mental math.

Mine was not harder mathematically, but the partner interrupted more and asked what the number meant immediately.

I would focus on not getting rattled. That was harder than the actual calculation for me.