Some people are standing quiet in a line, each person with a hat that has one of two colours. How many people can guess their colour correctly?

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A picture with a couple of people in a line, with different-coloured hats

Problem statement

Some people, let's say \(n\) people, are standing in a line. (Of course they are more than 2m away from each other, social distancing has to be taken seriously by all of us.)

Each person has a hat, like the picture above shows. Each hat is either light or dark, but no one knows the colour of their own hat and people can only look forward and cannot move at all. (Except perhaps to blink and to breath.)

Assuming they got a chance to meet before they got placed in a line and received their hats, what strategy do they have to agree on so that the most people can guess their own hat colour correctly? We can pretend that people who fail to guess their hat colour are sent to prison, and of course we want to keep as many people out of prison as possible. The only thing they know is that the hats will be distributed randomly, they have no idea how many hats of each colour will be distributed.

So your task is to devise the best possible strategy and to find out how many people that strategy saves, on average.

It is important to note that:

  • people cannot communicate with each other once they are in a line;
  • they can try to guess the colours of their hats in any order you see fit;
  • each person gets a single attempt;
  • everyone hears everyone's guess, but only the people behind the person making a guess know if that person got it right. Everyone else has no idea if the guess was correct or not.

Give it some thought...

If you need any clarification whatsoever, feel free to ask in the comment section below.


The solution to this problem will be posted here after this problem has been live for 2 weeks. You can also use that link to post your own solution in the comments! Please do not post spoilers in the comments here.

This problem was posed to me by my friend LeafarCoder.

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