Think in Odds

About Think in Odds

Think in Odds is a daily puzzle site about probability, published by David Z. Chen. The first puzzle, No. 001, appeared on 24 September 2026 with the classic question about the door you did not choose, and a new one has followed every day since. Each puzzle has a title, a topic such as Counting, Expected value, Strategy, Conditional probability or Bayesian reasoning, a difficulty of Easy or Medium so far, and an estimated time.

The question is short and plain, for example how many people you need for a better than even chance that two share a birthday, or how to split 50 green and 50 red balls between two jars to make a lucky draw most likely. A Read the assumptions panel spells out exactly what is meant, so there is one right answer. You type a number or a fraction and press Check answer; an optional Work it out box lets you write down your reasoning first.

If you are stuck, Give me a hint offers a nudge, and Reveal solution shows the full argument with illustrations. Each puzzle has its own page, a Challenge a friend link, and links to the previous and next puzzles. The archive lists every released puzzle, newest first, without spoilers, and can be filtered by topic. You can also sign up for a daily email.

The site is free and needs no account. It suits anyone who enjoys the Monty Hall problem and its relatives, students revising probability, and puzzle fans who like a single good question a day rather than a game to grind.

How to play Think in Odds

1. Open thinkinodds.com to see the day's numbered puzzle, its topic, difficulty and expected time.
2. Read the question, then open Read the assumptions to see exactly what is being asked.
3. Work out the probability, using the optional Work it out box to write down your reasoning.
4. Type your answer as a number or fraction and press Check answer.
5. Use Give me a hint if you are stuck, or Reveal solution to read the illustrated explanation.

Tips

• Always read the assumptions: many classic puzzles, like the two-children questions, change answer with one small condition.
• For at least one questions, work out the chance of none and subtract it from 1.
• For expected value puzzles, write the answer in terms of itself after the first step and solve the equation.
• With conditional and Bayesian questions, list every equally likely case first, then keep only the ones that fit what you were told.
• Fractions are accepted, so keep exact values rather than rounding.

Questions