Case Math Fundamentals

This lesson teaches the basic math habits that make case interview calculations accurate, clear, and interviewer-friendly.

Why This Matters

Case math is not advanced mathematics.

Most interview math uses:

• multiplication
• division
• percentages
• ratios
• simple breakeven or margin logic

But the pressure of the interview makes simple math harder. What interviewers often notice is not only whether you got the number right, but whether your setup was clear and your interpretation made sense.

What Good Looks Like

Good case math is organized math.

Strong candidates do three things well:

1. define what they are calculating
2. set up the equation cleanly
3. interpret the result in business terms

The arithmetic matters. The structure around the arithmetic matters just as much.

In the Interview

Whenever a numerical problem appears:

• say what you are solving for
• write or speak the formula clearly
• carry the units
• do the arithmetic in simple steps
• explain what the result means

This makes you look much more reliable, even if the calculation itself is simple.

A Reliable Way to Do It

1. Define the target number

What exactly are you trying to calculate?

Examples:

• total revenue
• margin
• number of customers
• profit per unit

2. Write the equation

For example:

• revenue = price x volume
• profit = revenue - cost
• margin = profit / revenue

3. Keep the units visible

Examples:

• dollars
• customers
• trips per year
• percent

4. Break the arithmetic into simple steps

Do not try to do too much at once.

5. Sanity check the answer

Ask:

• is the magnitude plausible?
• does the answer fit the business?

6. Interpret the result

A number without meaning is incomplete.

Example

Suppose the interviewer asks:

“If a company sells 2 million units at $15 each and variable cost is $9 per unit, what is contribution profit?”

A clean setup:

• revenue = 2 million x $15 = $30 million
• variable cost = 2 million x $9 = $18 million
• contribution profit = $30 million - $18 million = $12 million

Then interpret:

“So the company generates $12 million of contribution before fixed costs.”

What Strong Candidates Do

A strong candidate:

• states the formula
• uses clean arithmetic steps
• keeps track of units
• pauses to sanity check
• explains what the number means

What Weaker Candidates Tend to Do

A weaker candidate often:

• starts calculating without defining the target
• loses units
• compresses too many steps
• gets the number but gives no interpretation

Common Traps

Mistake 1: Arithmetic without structure

Messy setup creates avoidable mistakes.

Mistake 2: Forgetting units

Many errors come from mixing dollars, percentages, and counts.

Mistake 3: No sanity check

An implausible answer should trigger a quick re-check.

Mistake 4: No business meaning

The interviewer is not scoring you on calculator behavior. They are scoring you on business problem solving.

Practice Prompt

A service company has:

• 50,000 customers
• average annual spend of $120
• average service cost of $70 per customer

Calculate revenue and contribution profit, then explain what those numbers tell you.

Example Response

Here is a useful verbal pattern:

I want to calculate contribution profit. I will first calculate revenue, then variable cost, then subtract the two. Revenue is... Variable cost is... So contribution profit is... That tells us the business generates this amount before fixed costs.

What to Remember

• Strong case math starts with a clear target and a clear equation.
• Keep units visible and break arithmetic into simple steps.
• Sanity-check the result.
• Always explain what the number means in business terms.