15 Case Math Mistakes That Kill Offers (+ Fixes)

Smart candidates fail case interviews not because they can't do math, but because of avoidable mistakes. Here are the 15 most common errors and how to fix each one.

Why Candidates Fail on Math They "Know"

Pressure changes everything. In a calm environment, you can calculate 15% of 80. Under interview stress, with someone watching, you might say 8 instead of 12.

The solution isn't knowing more math—it's building systems that prevent mistakes.

Setup Mistakes

Mistake 1: Calculating the Wrong Thing

Pattern: Interviewer asks for profit margin, you calculate gross profit.

Example:

• Asked: "What's the profit margin?"
• You calculated: $50M profit
• Should have calculated: 50M / 200M = 25%

Fix: Before calculating, say: "Just to confirm, you want the margin as a percentage, correct?" Restate the question.

Mistake 2: Missing a Key Driver

Pattern: You analyze revenue but forget costs have multiple components.

Example:

• Asked: "Why did profit decline?"
• You analyzed: Revenue (down 5%)
• Missed: COGS up 8%, SG&A up 12%

Fix: Always list all components before diving into one. "Profit is revenue minus COGS minus SG&A. Let me check each."

Mistake 3: Wrong Formula

Pattern: Using ROI formula when asked for payback period.

Example:

• Asked: "How long until we recover our investment?"
• You calculated: (Gain - Cost) / Cost = ROI
• Should have: Cost / Annual Return = Years

Fix: Memorize what each formula answers. ROI = "How much return?" Payback = "How long?"

Arithmetic Mistakes

Mistake 4: Sign Errors

Pattern: Subtracting when you should add, or vice versa.

Example:

• Costs decreased by $5M
• You calculated: $100M - $5M = $95M total impact
• Should be: -$5M costs = +$5M profit

Fix: Write out the equation. Say "costs down means profit up" before calculating.

Mistake 5: Decimal Shifts

Pattern: 15% becomes 0.015 instead of 0.15

Example:

• Calculate 15% of $80M
• Error: $80M × 0.015 = $1.2M
• Correct: $80M × 0.15 = $12M

Fix: Convert percentages to fractions. 15% = 15/100 = "about 1/6" = ~$13M. Sanity check.

Mistake 6: Order of Operations

Pattern: Adding before multiplying when you shouldn't.

Example:

• Calculate: 5 + 3 × 4
• Error: 8 × 4 = 32
• Correct: 5 + 12 = 17

Fix: Use parentheses in your written work. Say steps out loud.

Communication Mistakes

Mistake 7: Silent Calculation

Pattern: Going quiet for 30 seconds while calculating.

Impact: Interviewer doesn't know if you're stuck or thinking.

Fix: Narrate your approach. "So I need breakeven units. That's fixed costs divided by contribution margin. Let me calculate..."

Mistake 8: Not Rounding Out Loud

Pattern: Using 47.3 in your head, interviewer can't follow.

Example:

• You think: 47.3 × 23 = 1087.9
• Interviewer hears: silence, then "about 1,100"

Fix: Say: "I'll round 47 to 50 for simplicity. 50 times 23 is 1,150."

Mistake 9: Skipping Sanity Checks

Pattern: Accepting obviously wrong answers.

Example:

• Calculate market size: $500 billion for dog food in Germany
• Should flag: That's $6,000 per person. Something's wrong.

Fix: Always do a quick per-capita or per-unit check. Does this make sense?

Business Logic Mistakes

Mistake 10: Wrong Unit

Pattern: Mixing up millions and thousands, or dollars and percentages.

Example:

• Revenue: $500M, Costs: $450M
• You say: "Profit is $50"
• Should say: "Profit is $50M" or "Profit is $50 million"

Fix: Always state units. Write M for millions. Double-check final answer includes units.

Mistake 11: Wrong Base for Percentages

Pattern: Calculating percentage change from wrong base.

Example:

• Price increased from $80 to $100
• Error: $20/$100 = 20% increase
• Correct: $20/$80 = 25% increase

Fix: Percentage change uses OLD value as base. New vs old? Base is old.

Mistake 12: Confusing Percentage Points and Percent

Pattern: Saying "increased by 5%" when you mean "increased by 5 percentage points"

Example:

• Market share went from 20% to 25%
• Error: "increased by 5%"
• Correct: "increased by 5 percentage points" or "increased by 25%"

Fix: Use "percentage points" when talking about difference between two percentages.

Mistake 13: Ignoring Time Dimension

Pattern: Comparing metrics without time normalization.

Example:

• Project A: 30% ROI over 3 years
• Project B: 30% ROI over 6 years
• Error: "They're the same"
• Correct: A = 10%/year, B = 5%/year. A is better.

Fix: Always annualize for comparison.

Mistake 14: Revenue vs. Profit Confusion

Pattern: Using revenue when profit is needed.

Example:

• Marketing spend: $1M, Generated revenue: $1.5M
• Error: "ROI is 50%!"
• Correct: If margin is 20%, profit = $300K. ROI = -70% (loss!)

Fix: ROI uses profit, not revenue. Always calculate profit first.

Mistake 15: Assuming Linear Relationships

Pattern: Assuming 2x input = 2x output.

Example:

• "If we double marketing, we'll double sales"
• Reality: Diminishing returns often apply

Fix: Ask: "Is there a reason to expect linear scaling here?"

A Simple Error Checklist

Before stating any answer:

☐ Did I answer the question asked?

☐ Units correct?

☐ Base correct for percentages?

☐ Sanity check passed?

☐ Did I state assumptions?

How to Integrate This with Practice

During practice sessions:

1. Keep an error log
2. Categorize each mistake (setup, arithmetic, communication, logic)
3. Focus next session on your most common error type
4. Celebrate when you catch yourself before making an error

Mistakes are learning opportunities. Track them, fix them, win.

Practice now → Mental Math Sprint