Puzzles

-Alien’s blood: You collected blood samples from 80 aliens. One of them carries a virus which is deadly to earth animals, infected individual would surely die within one day. You can inject these blood samples into rabbits in the lab. Given two days, what is the minimal number of rabbits needed to pin down the sample with the deadly virus. Uninfected alien’s blood sample do nothing to rabbits and you can inject any number of blood samples into a single rabbit each day.
(This is a variant puzzle I created based on a puzzle from a friend. The original puzzle only allows you to perform the experiment in one day.)

Answer: 4

-Prisoner’s hat: The warden designed a game for 50 prisoners. They have to line up back-to-front facing one direction wearing a hat that is one of the 7 colors of the rainbow. Each one can only see the hats of the prisoners in front of him and cannot see his own hat. The game starts with the first prisoner, who can see 49 hats in front of him, guessing the color of his own hat. Then the second prisoner and so on. Prisoners who guess their hat color correctly will be freed, otherwise will be executed. They can hear guesses from all the prisoners behind them but are not allowed to communicate during the game. Before playing the game, they can discuss the strategies. What is the optimal strategy and the maximal number of guaranteed survival?

Sam S. C. Wong

Memories

Puzzles