Press Start and type each answer: big round numbers, percentages of totals, growth and splits.
per min90 × 40Press Start, then type or tap the answer. No Enter needed. The clock starts when you press Start.
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Multiply the front digits, then add the zeros: 40 × 70 is 4 × 7 = 28 with two zeros, 2,800.
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1 min of questions. An answer is checked as soon as it has enough digits. The result is correct answers per minute; a wrong answer counts against accuracy.
This case interview mental math practice drills the sums a case question asks out loud: 300 × 60, 15% of 4,800, 3% growth on 5,200, a total split six ways. Type each answer; the score is correct answers per minute.
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Zeros first, then the digits
Most slips in case math are a lost zero. Round-number products trains one habit: multiply the front digits, then count the zeros, so 300 × 60 is 18 with three zeros, 18,000.
Percentages and growth
Percentages of big totals works from 10%: drop a zero, then scale. 1% to 4% growth drops two digits instead, the step behind any small growth rate. Full case math circuit mixes a product, a percentage, growth and a split.
Say the unit out loud, 42 thousand, not four two zero zero zero: it keeps the zeros honest.
Questions and answers
What mental math comes up in case interviews?
Mostly four kinds: multiplying round numbers, taking a percentage of a large total, a small growth rate, and dividing a total into shares. Each is one group of drills here, and every answer is a whole number.
How is this different from everyday mental math practice?
The numbers are bigger and rounder. Mental math practice drills sums up to about a thousand; these run into the tens of thousands, where the skill is keeping zeros and percentages straight rather than carrying.
How long should a practice run be?
Pick one run length and keep it, so your answers per minute can be compared from day to day. A 1-minute run fits between other preparation; a 5-minute run shows whether accuracy holds when you tire.
All 13 drills
Round-number products 5
Hundreds times tensFront digits, then count every zero: 300 × 60 is 3 × 6 = 18 with three zeros, 18,000.
Tens times tensMultiply the front digits, then add the zeros: 40 × 70 is 4 × 7 = 28 with two zeros, 2,800.
Thousands times a digitSay it in thousands: 7,000 × 6 is 42 thousand, 42,000. Keeping the unit in words stops a zero going missing.
Two digits times hundredsThe same with two zeros: 48 × 300 is 48 × 3 = 144, then 14,400.
Two digits times round tensDrop the zero, multiply, put it back: 36 × 40 is 36 × 4 = 144, then 1,440.
Percentages of big totals 4
1% to 4% growth1% is the total with two digits dropped: 3% of 5,200 is 52 × 3 = 156. That is one year of growth at 3%.
5% and 15% of a total5% is half of 10%, and 15% is 10% plus 5%: 15% of 4,800 is 480 + 240 = 720.
10% to 40% of thousandsFind 10% by dropping a zero, then multiply: 30% of 2,400 is 240 × 3 = 720.
Quarters of a big numberHalve, halve again: 25% of 3,600 is 900, and 75% is three of those, 2,700.
Splitting a total 2
Divide by 12Months in a year: 1,860 ÷ 12 is 1,200 ÷ 12 = 100, then 660 ÷ 12 = 55, so 155 a month.
Split a total 2 to 9 waysSplit the big part first: 2,436 ÷ 6 is 2,400 ÷ 6 = 400, then 36 ÷ 6 = 6, so 406.
Mixed case math 2
Full case math circuitOne of each kind in turn: a product, a percentage, growth and a split.
Market-sizing warm-upRound-number products and quarters, mixed, the way a sizing question chains them.