New White House Technology Officer Poses A Brain Teaser

Help Alice and Bob out of a jam

Coin Flip

Coin Flip

Redwood Photography, via Flickr CC BY-ND 2.0

A week ago Monday, Princeton professor Ed Felten accepted a job at the White House as Deputy Chief Technology Officer. It’s a long title for a simple mission: translating complex questions about technology into useful advice for the president. How did Ed end the first week on the job? Writing up a brainteaser, and asking people to answer it on Twitter.

Alice and Bob are playing a game. They are teammates, so they will win or lose together. Before the game starts, they can talk to each other and agree on a strategy. When the game starts, Alice and Bob go into separate soundproof rooms – they cannot communicate with each other in any way. They each flip a coin and note whether it came up Heads or Tails. (No funny business allowed – it has to be an honest coin flip and they have to tell the truth later about how it came out.) Now Alice writes down a guess as to the result of Bob’s coin flip; and Bob likewise writes down a guess as to Alice’s flip. If either or both of the written-down guesses turns out to be correct, then Alice and Bob both win as a team. But if both written-down guesses are wrong, then they both lose. The puzzle is this: Can you think of a strategy Alice and Bob can use that is guaranteed to win every time?

Felten is tweeting out hints. Here's the first one, and now that the President is on twitter, maybe he can give it a shot.