The best strategy is to make the calculation easy to follow before you make it fast. Start by stating what you are solving for, write the formula or relationship clearly, and then break the arithmetic into manageable steps.

Candidates often lose time because they try to do too much mentally while speaking. Use your paper. Keep units visible, separate intermediate calculations, and tell the interviewer what you are doing. Clear setup reduces both mistakes and cognitive load.

Once the setup is clean, speed comes from practice with recurring case patterns: percentages, margins, growth rates, weighted averages, and division. You do not need tricks for every number. You need enough fluency that the common operations stop consuming all your attention.

Finally, always connect the answer back to the case. The number is not the finish line. A concise interpretation shows that you understand why the math mattered in the first place.

My biggest improvement came from writing the setup before touching the numbers.

Do you usually make the mistake in the arithmetic or in setting up the equation? Those are pretty different problems.

Talking through the math actually slowed me down at first, then made me way more consistent.

Sanity checks save me constantly.