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084Quickly, without paper: what is 17.5% of 640? And what is 12.5% of Rs 4,480 crore?Oaktree Capital ManagementLos Angeles · 2022Oaktree Capital ManagementLos Angeles · 2022
Try it first
Which is the fastest route to 12.5% of 4,480?
Show the worked solution
112, and Rs 560 crore. For 17.5%, build it from blocks: 10% of 640 is 64, 5% is half of that, 32, and 2.5% is half again, 16; 64 + 32 + 16 = 112. For 12.5%, spot the fraction: it is one eighth, so halve 4,480 three times, to 2,240, 1,120 and 560. Check each with a second route before you say it.
How do you take an awkward percentage of anything in your head?
When a restaurant bill is Rs 640 and you want to tip 17.5%, nobody multiplies by 0.175. You find 10%, which is just moving the decimal point, and then cut pieces off it. Any percentage can be built from 10%, 5% (half of 10%), 1% and their halves, and each block is one easy operation on the last. For 17.5% of 640: 10% is 64, 5% is 32, 2.5% is 16, and the three add to 112.
Check with a second route: 20% is a fifth, 128, and 17.5% is 2.5 points less, so 128 minus 16 is 112. Two routes agreeing is what lets you say a number aloud with confidence in an interview where speed is being watched.
Seventeen and a half per cent of 640 is three easy blocks, 64, 32 and 16, adding to 112, and twelve and a half per cent of 4,480 is one eighth, reached by halving three times to 560. Which percentages are fractions in disguise?
Some percentages are clean fractions, and dividing is faster than building. 12.5% is one eighth, so 12.5% of Rs 4,480 crore is three halvings: 2,240, 1,120, 560. The same idea gives 37.5% as three eighths (560 x 3 = 1,680) and 87.5% as the whole less one eighth (4,480 minus 560 = 3,920). Desk interviewers like these because they turn up constantly in fee splits, stake sizes and tax at round rates.
Percentage Fraction Fastest move 12.5% 1/8 halve three times 37.5% 3/8 one eighth, times three 62.5% 5/8 half plus one eighth 87.5% 7/8 whole less one eighth 16.67% 1/6 halve, then divide by 3 8.33% 1/12 one sixth, halved 6.25% 1/16 halve four times 2.5% 1/40 10%, then quarter it Percentages worth knowing as fractions, with the quickest mental route for each. What makes a mental answer trustworthy under time pressure?
Two habits. First, estimate before computing: 17.5% is a little under a fifth, so the answer must be a little under 128. If your arithmetic produces 1,120 or 11.2, the estimate catches the slipped decimal at once. Second, keep the units: the second answer is Rs 560 crore, not 560. An interviewer marks a right number with units and a sanity check far above a right number mumbled after a long silence. Speak the route as you go, so a slip is visible and recoverable.
Where candidates lose it
The common slip is multiplying digit by digit, 640 x 0.175, losing the decimal and answering 1,120 or 11.2. Building from 10% keeps every intermediate number sensible, so a misplaced decimal is obvious before you speak.
The second loss is dropping the unit or answering slowly in silence. Say 'Rs 560 crore', say the route out loud, and add the check: one eighth of roughly 4,500 is about 560. The interviewer is testing fluency with the numbers a desk uses every day, not long multiplication.
What the interviewer asks next
- What is 37.5% of 2,400? And 6.25% of 1,280?
- A Rs 360 crore fee is split 15% and 85%. What are the two pieces?
- What is 7% of 2,350, and what is your second route to check it?
Asked at Oaktree Capital Management, Generalist, Los Angeles, 2022 (Wall Street Oasis):
Quick mental math questions are unexpected. Was asked around 6 of them.
Asked at Oaktree Capital Management, Generalist, Los Angeles, 2022 (Wall Street Oasis):1 with Asso mostly technical, 1 with VP mostly quick math
098Without paper or a calculator: what is 301 x 447?General AtlanticNew York · 2026
Try it first
What is the first move?
Show the worked solution
134,547. Split 301 into 300 and 1. Three hundred times 447 is 447 x 3 with two zeros: 1,341, then 134,100. Add one more 447: 134,547. Check before you say it: 300 x 450 is 135,000, so the answer should sit a little below that; the last digit must be 1 x 7, a 7; and the digit sums agree, since 301 reduces to 4, 447 to 6, 4 x 6 = 24 reduces to 6, and 1+3+4+5+4+7 = 24 also reduces to 6.
Why split 301 rather than attack both numbers?
If a shop sells 301 items at Rs 447 each, nobody in the shop multiplies 301 by 447 digit by digit. They price 300 items and add one more. Break the awkward factor into a round number and a small remainder, multiply the round part first, and the remainder is one easy addition. 447 x 3 is 1,200 plus 141, which is 1,341; two zeros make it 134,100; plus 447 is 134,547. Say each intermediate number aloud as you reach it, so the interviewer can follow and a slip is caught before it compounds.
The split can go the other way too. 447 is 450 less 3, so 301 x 447 = 301 x 450 minus 301 x 3. The first is 300 x 450 plus 450, 135,450; the second is 903; the difference is 134,547. Having two routes is what turns a hopeful answer into a checked one.
The rectangle of 301 rows of 447 is a round block of 300 rows worth 134,100 plus a single row worth 447, so the product is 134,547, and the estimate, the last digit and the digit sums all agree with it. How do you check an answer you cannot write down?
Three checks, each a few seconds. The estimate: 300 x 450 is 135,000, and you used one factor slightly above and one slightly below, so the answer should be close to it, which 134,547 is. The last digit: 1 x 7 ends in 7, and so does the answer. The digit sums, an old bookkeeper's test: reduce each number to a single digit by adding its digits, 301 gives 4 and 447 gives 15 then 6; the product 24 reduces to 6, and the answer's digits 1+3+4+5+4+7 = 24 also reduce to 6. A wrong answer rarely survives all three checks, and running them aloud shows the interviewer a habit, not a lucky number. The digit-sum test has a blind spot: it misses errors that are exact multiples of 9, including two digits swapped, which is why the estimate sits beside it.
Product Split Answer 301 x 447 300 x 447 + 447 134,547 299 x 447 300 x 447 - 447 133,653 25 x 447 447 x 100 / 4 11,175 447 x 11 4,470 + 447 4,917 447 x 99 44,700 - 447 44,253 Products that look hard become one round multiplication plus one addition or subtraction once the awkward factor is split. What is the interviewer marking?
Pace, an audible route and a check, in that order. A right answer after a long silence scores below a right answer reached in three spoken steps, because the desk wants to hear how you handle a number, not whether you can. The limitation is that this trick suits factors near a round number. For something like 347 x 683, say that you would estimate, 350 x 700 less a bit, give about 237,000, and label it an estimate; pretending to exact arithmetic you cannot check is the real failure.
Where candidates lose it
The common failure is attempting long multiplication in the head, losing a carry, and producing a number like 134,447 or 133,547. Holding four partial products at once is what paper is for. Splitting 301 into 300 and 1 leaves two numbers to hold, 134,100 and 447, and nothing to carry.
The second loss is giving 134,547 with no check. Say the estimate and the last digit as you go; the interviewer is listening for the habit of verifying, which is the same habit that catches a misplaced decimal in a model.
What the interviewer asks next
- What is 299 x 447, and what is 301 x 453?
- What is 447 x 25 without multiplying?
- Give an approximate answer for 347 x 683 in five seconds, and say how far off it might be.
Asked at General Atlantic, Generalist, New York, 2026 (Wall Street Oasis):
what is 301x447
