About

Every problem here is transcribed by hand from Introduction to Probability (second edition) by Joseph K. Blitzstein and Jessica Hwang — the Stat 110 textbook. The problems are theirs; the transcription, and any slips in it, are mine. Once you’re signed in, the Report an issue link under every problem is the fastest way to get one fixed.

There are no solutions on this site. The authors publish selected solutions themselves, and stat110.net has the lectures and much more.

Status is per part. You mark each part of a problem; the problem’s own status follows from them. Every part solved → Solved; any part at Needs review → the whole problem; nothing touched → Not started; anything else → Attempted. The dashboard counts and Random Problem both work off this, so a problem with one unsolved part stays in the pool.

Random Problem never repeats finished work. It picks evenly among problems you haven’t solved — solved ones are excluded outright. On the Problems page it draws from whatever you’ve filtered to, which is the quickest way to drill one subtopic. If everything matching is already solved, it allows solved problems rather than sending you somewhere unrelated.

Math in notes. $x^2$ is inline math — the closing $ is added for you, and it renders when the cursor leaves. $$…$$ is a centered block. \ inside math searches ~230 commands; \$ is a literal dollar sign. Click a rendered formula to edit it (Esc reverts), select text and press $ to wrap it, and Tab hops out of a formula. 1. , - and a) start lists. The ? beside Notes on any problem keeps this list.

The distribution table is one click away. The little histogram beside Notes opens the book’s Appendix C reference above your notes: one distribution at a time, with its PMF or PDF, support, mean and variance in the book’s own parametrizations. A second click opens the full table. Hover any formula to copy its LaTeX.

References are links. When a problem says see Exercise 4, that’s a link — the book’s exercises are these problems. References to an Example, Theorem or Section point into the book itself, so those stay plain text.

Hidden behind a hover. Hover the Problem N title for Copy problem and + notes — both copy clean Markdown with formulas as $…$ source, ready for your own notes, an email, or an AI chat. Hover Random Problem on a problem page to choose between any chapter and the current one. On a touch device, tap instead.

Keyboard. j and k step to the next and previous problem; / jumps to search on the Problems page. All three are ignored while you’re typing.

Links you can share. The Problems page keeps its filters in the address bar, so a filtered view is a real link — bookmark it or send it. Every problem has its own URL too.

Getting back to something. Recently viewed on the Problems page switches the table to what you’ve opened, newest first. The dashboard’s Continue Studying lists what you’ve started but not finished.

Hints and spoilers. The book’s hints sit behind Show hint, for everyone — a hint only helps once you’re stuck. In Settings you can also hide the chapter and topic on problem pages, since knowing a problem’s topic often gives away the approach.

Signed out, everything stays in this browser. Signing in is optional and limited to Harvard Google accounts; it saves your work to the site’s database so it’s available on any device. Anything done before signing in is uploaded the first time you do.

Changes are saved to this browser immediately, even offline, and to the database within a few seconds. If a save fails, the account menu says so, and it retries once you’re back online.

When you’re signed in, the database stores:

• Account: your name and email address from Google, which identify your account.
• Progress and notes: the status of each part and your notes, plus a few recent copies of each note as a backup against sync errors.
• Settings: your choices in Settings, such as hiding the chapter and topic, so they apply on every device.
• Activity: which problems you open, when statuses change, and how long each problem is open while its tab is in view. This is what Recently viewed and the Statistics page are built from.
• Reports: any problem reports and feedback you send, with your email, so you can be contacted about them.

In Settings, you can download a copy of all of it, or delete your account, which permanently erases everything.

The site works by keyboard throughout — every control is reachable with Tab, chart squares included, and the tour can be driven with the arrow keys. Formulas carry MathML alongside the rendered math, so screen readers read the formula rather than a picture of one. Animation respects your system’s reduced motion setting.

Corrections to a problem belong on that problem, where the context is. Anything about the site itself — a feature request, or something broken or confusing — goes here.