Probability · Brainteasers · Mental math · Markets

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Practice 1,200+ real quant-interview questions: probability, brainteasers, mental math, options and market-making. Real questions asked at top trading firms and hedge funds. Full worked solutions, hints, and timed mock interviews.

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Jane Street Hard Probability

Forming a Triangle

A stick of unit length is randomly broken into three pieces. Assuming each break follows a uniform distribution along the stick, what is the probability that the three segments can form a triangle?

Your answer…
Graph the sample space in two dimensions. Let x and y be the two break points, then work out which region of the unit square …
🔒 Full solution
SIG Jane Street Citadel Goldman Sachs Five Rings Akuna Capital Optiver DRW IMC Virtu Financial Hudson River Trading Two Sigma DE Shaw Chicago Trading Company Maven Securities Flow Traders GSA Capital Jump Trading Mako All Options Da Vinci Wincent

Questions asked at firms like

01 · The bank

1,205 dice. One for every question.

Every die is a question in the bank: from interview debriefs, the firms' online assessments, and the classic problems interviewers reach for. Point at any die to read it.

02 · Five topics

Sorted by topic.

03 · Three levels

Start easy, then work up.

04 · 38 firms

Drill the firm you are interviewing with.

Real questions

Asked in real interviews and online assessments.

553 questions are tagged with the firms that asked them, from interview debriefs and the firms' own online tests. The rest are the classics those interviews are built on.

Circular PartitionMediumFrom SIG interview · asked at SIG, Virtu Financial
2 LeadEasyFrom Citadel online assessment · asked at Citadel, Optiver, SIG
Clarence's BreadHardFrom Old Mission interview · asked at Old Mission
Meeting on a GridMediumFrom Optiver online assessment · asked at Optiver
Game Show IIIMediumFrom Akuna interview · asked at Akuna, Squarepoint Capital

The working

Every answer comes with the reasoning.

All 1,205 questions have a hint and a full worked solution, so when you are stuck you get a nudge first, and when you are wrong you see exactly where.

Question

Game Show III

Ten doors, one $\$1000$ prize. You pick one, the host opens three goat doors, and you may switch. What is the game worth?

Hint

Which strategy should you go into this game with?

Solution

Always switch. $\frac{9}{10}$ of the time your first pick is a goat, and switching leaves $6$ closed doors, one with the prize: $\frac{9}{10}\cdot\frac{1}{6} = 0.15$.

Answer

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One of the 15 questions with a free solution

Pricing

Thirty questions free.
The whole bank for £20 a month.

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Explore

Seventeen tracks, each one in order.

Hand-picked playlists through the bank. Pick a track and work it top to bottom: warm-ups first, the hard ones last.

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Who built this

One bank, instead of scattered PDFs

Built by a Cambridge ML grad and physicist with real trading desk experience. Sick of interview prep being scattered across forums, PDFs, and half-dead threads. If you want the internship or the job, grinding these questions is genuinely how you get there.

FAQ

Before you subscribe.

The nine questions people ask most. Anything else, write to contact@quantinterview.io.

Are these real interview questions, or made-up practice problems?

Real ones. 553 of them are tagged with the firms that asked them, sourced from interview debriefs and people taking the online assessments, and the rest are the classic problems those interviews are built on. Firms do rotate their questions over time, so the point is not to memorise the exact prompt you will get. It is to drill enough of the real patterns so that whatever they throw at you, you have seen enough similar questions that you are fast and calm.

Is it firm-specific? Can I filter by the company I'm interviewing with?

Yes. You pick the firm and practise the questions that firm has actually asked. We tag 38 firms, so if you have a Jane Street or Optiver or DRW loop coming up, you can go straight at their style instead of grinding generic problems.

How is this different from Green Book / QuantGuide / QuantQuestions / TraderMath?

Green Book is great and you should do it, but it is a general textbook, not firm-specific and not a live, growing bank. The paid sites each do one slice: some are mostly mental-math OA drills, some have a few hundred questions. This is the biggest bank of the lot, it is firm-taggable, and it is the cheapest of them. If you are only going to pay for one thing, the honest pitch is that this covers the most ground for the least money.

Has anyone actually got an offer off the back of this?

Yes. People who prepped on these questions have gone on to trade at IMC, Da Vinci, DV Trading, Optiver, Maven, Wincent and others. These are real offers real students have received.

When should I start prepping? I don't have an interview yet.

Start now. Realistically you need one to two months of steady practice to be sharp, not two weeks. The single most common mistake I see is people who wait until they receive an interview invite and then start prepping, by which point it is already too late. If you are a first or second year, being ready before you apply is the whole edge.

Do the questions get outdated since firms change them?

Firms rotate, yes, and that is exactly why volume matters and why we keep adding. You are not buying a fixed list to memorise. You are building the pattern recognition and speed that survive a rotation, plus we keep the bank current.

Is it worth it for the price?

It is £20/month, less than two meals out, and one offer pays for it several thousand times over. First-subscription nerves are normal, but the risk is not the price, it is spending the summer on random PDFs and walking into a Jane Street final under-prepared. Plus, you can cancel anytime.

Can I cancel anytime?

Yes. You will also keep access to the questions and answers until the end of your active subscription period.

I've never done quant prep before. Will I even understand the questions, or do I need to be advanced already?

You are the person who needs this most. Easy questions let you get an understanding and practise the most important core topics (expected values, probabilities, combinatorics, recurrence relations, game theory) that come up most often. Then you move on to mediums with better intuition, and grind baby grind.

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