Welcome.
Small sessions, every day. That's how quant interviews are won.
Mental Math
Arithmetic under the clock: the entry ticket to every trading interview. Five formats, from untimed practice to the two screens firms actually run: the typed 80-in-8 and the multiple-choice 60-in-6, plus a trainer for the shortcuts that beat doing the sum.
Number Sequences
"What comes next?" is its own section at Optiver, IMC and Flow Traders, and it rewards a different reflex than arithmetic: read the gaps, read the ratios, then commit. 26 pattern families across three tiers, and every miss tells you the rule you did not see.
Probability Lab
The questions that actually get asked (Bayes, expected value, combinatorics, ruin, strategy) with full worked solutions.
Likelihood-list
Rank the outcomes from most to least likely, against a per-question clock. No arithmetic to type, just a sharp sense of the odds.
Brainteasers
Wrestle before you peek. Hints cost a little edge; the reveal costs all of it.
Fermi Lab
Decompose, estimate, sanity-check. Within 3× of reference is a win.
Market Maker Arena
Quote a two-sided market on a hidden quantity. Earn the spread, dodge the sharks, respect your inventory.
Arbitrage
A board of two-sided quotes hides one risk-free trade. Buy the cheap side, sell the rich side, net every good to zero, and walk away with a positive P&L.
Interview Sim
Twenty minutes, three timed sections, one verdict. Sit it like it counts.
Prep by Firm
What each firm actually tests, mapped onto this site's desks. Train the exact formats.
Analytics
Where your edge comes from, and where it leaks. Weakest categories float to the top.
Leaderboard
Every desk on one tape. Edge is earned, not given. Climb.
The Playbook
Results worth knowing cold. Skim before an interview; internalize over time.
Probability toolkit
- Law of total probability: P(A) = Σ P(A|Bᵢ)·P(Bᵢ). Condition on the first step; most "expected time" puzzles fall to this.
- Bayes: P(H|E) = P(E|H)·P(H) / P(E). Posterior odds = prior odds × likelihood ratio. Never forget the prior.
- Linearity of expectation: E[X+Y] = E[X]+E[Y], no independence needed. Break nasty quantities into indicator variables.
- Indicators: E[1_A] = P(A). "Expected number of …" = sum of probabilities.
- Tail-sum: for non-negative integer X: E[X] = Σ P(X ≥ k). Kills E[max] and waiting-time questions.
- Variance rules: Var(aX+b) = a²Var(X); Var(X+Y) = Var X + Var Y + 2Cov(X,Y).
- Symmetry & exchangeability: "position k is an ace" = 1/13 for every k; "last ball is red" = fraction of reds. Look for it before computing.
- Memorylessness: geometric and exponential only. The past wait is sunk.
- Optional stopping: for a martingale with a bounded stopping time, E[X_T] = E[X₀]. Instant gambler's-ruin probabilities.
Numbers to know cold
| Quantity | Value |
|---|---|
| E[flips to first H] / to HH / to HT | 2 · 6 · 4 |
| E[rolls to first 6] | 6 |
| E[rolls to see all 6 faces] (coupon) | 14.7 = 6·H₆ |
| Birthday: P(match) > ½ | 23 people |
| E[max of 2 dice] / E[sum of 2 dice] | 161/36 ≈ 4.47 · 7 |
| Var of one die (σ) | 35/12 ≈ 2.92 (σ ≈ 1.71) |
| P(no fixed point), n large (derangement) | 1/e ≈ 36.8% |
| Secretary success / cutoff | 1/e · skip n/e |
| Gambler's ruin, fair: P(reach N from i) | i / N |
| E[max of n Uniform(0,1)] | n/(n+1) |
| E|X−Y|, X,Y ~ U(0,1) | 1/3 |
| Broken stick forms a triangle | 1/4 |
| Meeting problem (15 min in an hour) | 7/16 |
| Ballot: A always strictly ahead | (a−b)/(a+b) |
| 100 prisoners (cycle strategy) | 1 − ln 2 ≈ 31% |
| Heads in 100 flips: mean ± σ | 50 ± 5 |
| Kelly, even odds | f* = p − q |
Combinatorics identities
- Permutations / combinations: n!/(n−k)! ordered; C(n,k) = n!/(k!(n−k)!) unordered.
- Multiset permutations: n! / (n₁!·n₂!·…). MISSISSIPPI = 11!/(4!4!2!) = 34,650.
- Stars & bars: non-negative solutions of x₁+…+x_k = n: C(n+k−1, k−1).
- Inclusion–exclusion: |A∪B∪C| = Σ|A| − Σ|A∩B| + |A∩B∩C|; derangements Dₙ = n!·Σ(−1)ᵏ/k!.
- Catalan: Cₙ = C(2n,n)/(n+1): balanced brackets, monotone paths under the diagonal, binary trees. 1, 1, 2, 5, 14, 42…
- Pigeonhole: n+1 items in n boxes force a repeat. Guarantee ⇒ think worst case.
- Double counting / symmetry: count one thing two ways; or argue every position is exchangeable.
Mental math tricks
- Flip percentages: a% of b = b% of a. 8% of 25 = 25% of 8 = 2.
- ×5, ×25, ×50: ×10/2, ×100/4, ×100/2. And ÷5 = ×2/10.
- ×11: 43 × 11 → 4_(4+3)_3 = 473 (carry when needed).
- Squares near 50: (50±d)² = 2500 ± 100d + d². So 47² = 2500 − 300 + 9 = 2209.
- Squares near 100: (100±d)² = 10000 ± 200d + d².
- Difference of squares: a·b = ((a+b)/2)² − ((a−b)/2)². 18×22 = 20² − 2² = 396.
- Ending in 5: (10a+5)² = a(a+1) then append 25. 85² = 72_25.
- Decimals: count total decimal places: 1.2 × 0.04 → 12×4 = 48 → 0.048.
- Fractions to know: 1/8 = .125, 1/6 ≈ .1667, 1/7 ≈ .1429, 1/9 = .111…, 1/12 ≈ .0833, 1/16 = .0625.
- Rule of 72: years to double ≈ 72 / rate%. Handy for growth sanity checks.
- √2 ≈ 1.414, √3 ≈ 1.732, √5 ≈ 2.236, e ≈ 2.718, ln 2 ≈ 0.693, π ≈ 3.1416.
Market-making principles
- Quote around fair value: your mid should track conditional expected value given everything revealed so far.
- Spread prices two risks: variance of the underlying and the chance your counterparty knows more (adverse selection). More of either ⇒ wider.
- Getting filled is information: an instant, full-size lift means fair value is probably above your ask (winner's curse). Update, then re-quote.
- Skew for inventory: long ⇒ shift both quotes down to sell off; short ⇒ shift up. Manage risk with the mid, not only the width.
- Too wide never trades: no flow, no edge. Market making is selling liquidity; price it, don't hide it.
- Size like Kelly: edge/odds bounds your size; half-Kelly is the practitioner's comfort zone.
- Say your market out loud. In interviews: "I'm 17 at 20, five up" beats a silent number. Communicate width, level, and size.
Distribution cheat table
| Distribution | Mean | Variance | Use it for |
|---|---|---|---|
| Bernoulli(p) | p | p(1−p) | indicators |
| Binomial(n,p) | np | np(1−p) | #successes |
| Geometric(p) | 1/p | (1−p)/p² | waiting for first success |
| Poisson(λ) | λ | λ | rare-event counts |
| Uniform(0,1) | 1/2 | 1/12 | geometric probability |
| Exponential(λ) | 1/λ | 1/λ² | memoryless waits |
| Normal(μ,σ²) | μ | σ² | CLT; 68 / 95 / 99.7 |
Interview game plan
- Restate the problem in one sentence and confirm the setup: half the failures are answering a different question.
- Announce a plan before diving: "I'll condition on the first flip" earns points even before any algebra.
- Think aloud, cleanly: interviewers grade the path. Silence reads as being lost; narrated dead-ends read as process.
- Try small cases: n = 1, 2, 3 reveals the pattern in most puzzles.
- Sanity-check: units, bounds (probabilities in [0,1]), limits (p→0, n→∞), symmetry.
- When stuck: say what you know, what you'd try, and what feels wrong. Ask for a hint after real effort; using a hint well is a strong signal.
- Behavioral prep: have crisp stories for: a failure, a disagreement, a decision under uncertainty, why trading/research, why this firm. 90 seconds each.
- Own your resume: every line can be dissected. If you can't derive it, cut it.
The story
Who's behind the desk, and why it's built the way it is.
My Quant Journey is built and run by Florian Dinant-Meier, founder. It started as the training desk I wished existed while preparing for quantitative trading and research interviews: one place to drill mental math, probability, brainteasers, Fermi estimation and market-making, with feedback that teaches rather than a bare answer key.
The promise is simple, and it won't change: everything you train on is free, for good. The only paid layer, later, covers the AI features that cost real money to run each time they're used, priced at cost. No paywall on practice, no dark patterns, no auto-renewing trap.
Credits & contact
- Contact: contact@myquantjourney.app
- LinkedIn: florian-dinant-meier
- GitHub: FlorianDinant
Found a bug or have an idea? The contact address goes straight to me.
Privacy
What we store, where, and how to get it out or delete it. Last updated 15 July 2026.
As a guest, nothing leaves your device
Without an account, all your progress (XP, streaks, badges, session history, settings) lives only in your browser's localStorage. We never see it, and it never leaves your machine. Clearing your browser data erases it.
With a free account
If you create an account, we store your e-mail address, the username you choose, and your training progress and stats, hosted on Supabase in the EU (Paris). Purpose: signing you in, syncing progress across devices, and the leaderboard.
Public vs private. Your leaderboard entry (username, XP and level, accuracy, streak, best sprint) is public by design. Your e-mail and your saved progress are private and readable only by you, enforced by row-level security.
AI features
When you use the AI examiner, coach or grader, the text you submit is sent through our server to a language-model provider (Mistral, Groq or Google) to generate feedback, then returned to you. We don't keep your submissions beyond the request needed to answer them, and we don't use them to train any model. Each provider processes the request under its own policy.
Sign-in codes are sent through Resend (EU). Beyond those, the only e-mail we ever send is an optional weekly training debrief from the AI coach, and only if you switch it on in your account (it is off by default). Every debrief carries a one-click unsubscribe, and you can also turn it off anytime in your account. We never sell your address and send no other marketing. The beta Feedback button e-mails us your message when you hit Send (with your username and account e-mail if you're signed in, so we can reply); nothing is sent until you do.
Analytics
We use Cloudflare Web Analytics to count visits. It's privacy-first: no cookies, no fingerprinting, no cross-site tracking. It only tells us aggregate numbers like page views, where visitors came from and rough country, and we can't identify you from it. There's no advertising and no tracking cookies anywhere on the site. The other external requests are the web fonts, the Supabase API (for accounts) and the language-model providers (for AI feedback).
Your rights
You can export all your progress as a JSON file at any time (Settings then Backup), and delete your account from the account panel, which removes your profile and saved data from our servers. For anything else, write to contact@myquantjourney.app.
This notice is provided in good faith for a project run by an individual; it will be formalized as the project's legal entity is set up.
Terms of use
The short version, in plain language. Last updated 13 July 2026.
- What this is. A free tool to practice for quantitative interviews, provided as-is, for education and training, with no guarantee of accuracy or fitness for any particular purpose.
- Your account. You're responsible for activity under your account. Please don't try to game the leaderboard, script the AI endpoints or abuse the service; automated caps are in place and abuse may lead to a block.
- AI feedback. Answers and feedback from the AI features can be wrong or incomplete. Treat them as a study aid, not as professional, financial or career advice.
- Content. Questions, solutions and materials are for your personal training. Please don't scrape or redistribute them wholesale.
- Availability. Features may change, pause or be discontinued. The free AI features run under fair-use daily limits; any future paid features will come with their own terms.
- Contact. Questions about these terms: contact@myquantjourney.app.
A full set of terms and legal notices (with company details) will follow once the project's legal entity is registered. Governed by French law.