Cheryl's birthday puzzle: one knows the month, one knows the day, and three sentences give the date away.

Ten possible dates. Albert is told the month and Bernard the day. Albert says he doesn't know but knows Bernard doesn't either. Bernard says he now knows. Albert says he knows too. The story asks for the date. You type your answer and check it. Then you step through the answer and the breakdown.

Why quant interviews test this

Interviewers use it to see whether you can reason about what someone else knows, one statement at a time. The written solution is at /brain-teasers/cheryls-birthday/solution.

The Albert, Bernard and the Birthday guide covers how scoring works, the strategy that wins, a worked example and the mistakes most players make.

More Brain Teasers games

• Screwy Pirates - Five rational pirates vote on how to split 100 gold coins, and you work out the split by backward induction from a one-pirate crew up.
• Tiger and Sheep - A hundred rational tigers and one sheep on a magic island, where any tiger that eats becomes a sheep itself - solved by induction on parity.
• The Split Deal - The prisoner's dilemma played as a story: two thieves, separate rooms, and one offer each.
• Stag or Hare - The stag hunt as a story: two equilibria, and a choice that comes down to trust.
• One Offer, No Discussion - The ultimatum game as a story: one split of 100 pounds, one accept-or-reject, no negotiation.
• The Growing Pot - The centipede game as a story: a pot that doubles on every pass, and logic that says take it immediately.
• All Brain Teasers practice
• Every game guide