How to Handle Brain Teasers and Mental Math Without a Calculator

Many banks, including firms such as Evercore, ban calculators in certain interviews or assessments. Exact rules vary by firm, office, and recruiting cycle, so follow the instructions you receive rather than assuming a calculator will be allowed.

Mental math questions test whether you can work comfortably with numbers used in valuation and financial analysis. Brain teasers show how you break an unfamiliar problem into manageable steps. In both cases, interviewers usually care about your reasoning, accuracy, and composure, not whether you instantly produce a clever answer.

Do not force every calculation into one step. Break numbers apart, simplify fractions, and round only when the question permits an estimate. Write down intermediate results if paper is allowed. I would prioritize accurate decomposition over raw speed in practice.

Memorize common percentage and fraction relationships because they appear frequently in finance. For example, 10% is one-tenth, 5% is half of 10%, 25% is one-quarter, and 12.5% is one-eighth. You should also practice multiplication tables, percentage changes, and division involving zeros.

• For 18% of 250, calculate 20% of 250 and subtract 2%: 50 minus 5 equals 45.
• For 48 divided by 1.2, multiply both numbers by 10: 480 divided by 12 equals 40.
• For 99 times 37, calculate 100 times 37 and subtract 37: 3,700 minus 37 equals 3,663.

Suppose the interviewer asks: “A company has $240 million of revenue and a 15% EBITDA margin. What is EBITDA?” EBITDA means earnings before interest, taxes, depreciation, and amortization. Margin is EBITDA divided by revenue, so you need to calculate 15% of $240 million.

State your setup before you start calculating. This confirms that you understand the question and gives the interviewer a chance to correct a mistaken interpretation.

For estimation questions, define the population or market, divide it into logical groups, assign simple assumptions, and calculate from those assumptions. For logic puzzles, identify the rules, constraints, and necessary information before testing possible solutions.

Think aloud, but do not narrate every uncertain thought. A strong response sounds like this: “I will estimate the number of customers first, then usage per customer, and finally capacity per location.” If a key detail is unclear, ask a concise clarifying question. When no additional information is available, state a reasonable assumption and proceed.

1. Restate the objective.
2. Clarify ambiguous terms.
3. Choose a framework and assumptions.
4. Calculate in small steps.
5. Check whether the result is reasonable.

When a problem stalls, do not rush into a guess or go silent indefinitely. Give yourself a brief pause, summarize what you know, and identify the exact step blocking you. My preference is to pause briefly rather than fill the silence. A focused pause looks controlled, while scattered talking makes your reasoning harder to follow.

For mental math, return to the last intermediate result you trust and rebuild from there. For a brain teaser, try a simpler version of the problem or establish reasonable upper and lower bounds. This can reveal the missing relationship without forcing you to abandon the structure you already established.

If the obstacle is an unclear assumption, explain what is missing and ask whether the interviewer wants you to proceed with a stated assumption. If the interviewer offers a hint, acknowledge it and incorporate it directly. Do not apologize repeatedly or pretend the insight was yours. Use the hint, complete the reasoning, and give the clearest answer you can.

Practice in short sessions without a calculator, using percentages, multiplication, division, and finance-style word problems. Time yourself only after your method becomes dependable. Speed built on careless habits will not help you in an interview.

If you make an error, correct it directly: “I used 20% instead of 15%; let me revise that step.” Do not hide the mistake or abandon your structure. Before giving a final answer, check the sign, units, decimal placement, and approximate magnitude.

• Expect some banks and assessments to prohibit calculators, and prepare accordingly.
• Break calculations into simple percentages, fractions, and intermediate steps.
• For brain teasers, state your assumptions and use a clear framework instead of guessing.
• Communicate your reasoning and perform a quick reasonableness check before finalizing.