About

Elo Street is a free set of practice problems for quant researcher interviews: probability, random walks and Markov chains, order statistics, statistics and inference, linear regression, a little linear algebra, and a few puzzles. There are no ads, no paid tier and no account.

The daily problem. The home page shows one problem a day, the same for everyone, and a new one at midnight. Solve it before opening the solution to keep your streak going, then share your result. All the problems are also available at any time in the full list.

How it works. Each problem has a short statement, optional hints shown one at a time, and a worked solution. Type your answer and press Enter. A problem counts as solved if you answer it correctly before opening the solution. If you open the solution first, it is marked as seen.

Answers. Most answers are numbers. You can type a fraction (11/18), an expression (sqrt(2)/2, e^-1, pi/4, 2ln(2)) or a decimal. A decimal is accepted if it is the exact answer rounded or truncated, with at least 3 significant digits: for $2/3$, 0.667 and 0.666 are accepted, 0.67 is not precise enough.

The problems. They are written for this site. Several are built on well-known ideas, since interviews reuse them, but the statements and solutions are written from scratch. Every answer is checked by an automated test, either by exact computation or by a simulation with a fixed seed.

Your data. Without an account, your progress (solved problems, attempts, hints used, time spent) is saved in this browser's local storage and never leaves your device. The stats page can export it to a file and import it back.

Accounts. An account is optional. It keeps your progress and your daily streak on all your devices and puts your streak on the leaderboard. With an account, your email address, your username and your progress are stored with our database provider, Supabase. Only your username and your streak numbers are public. You can delete your account at any time from the account page; this deletes everything stored with it.

The JetBrains Mono font is loaded from Google Fonts.

Shortcuts. On the list: / to search, j and k to move, Enter to open. On a problem: Enter to check, h for the next hint, s to reveal the solution, n and p for the next and previous problem, Esc to go back.