State the setup briefly when it helps the interviewer follow your logic, but you do not need to narrate every trivial arithmetic step.

A concise formula or equation makes mistakes easier to catch and shows that the answer comes from a structured approach.

For a margin calculation, saying 'profit divided by revenue' before computing may take two seconds and makes your logic clear.

Build the habit of stating the equation and units once, then calculate efficiently. If useful, this guide on case interview mental math gives more detail.