3 guards, 2 doors…
Solution to the riddle!

Our participation at Spark Summit 2017 in San Francisco was once again a great experience. Not only because of the quality of the speakers, but the special atmosphere built up by Spark lovers. In order to enliven the breaks between sessions talks, we brought a riddle, inviting all attendees to solve it.

Participation was high and we had all kinds of answers but… just one answered correctly! For those keen to find out the answer, read the next lines carefully.

Riddle_3guards-2doors

 

The right questions to the right guards

  1. How do we figure out the right questions?
  2. The complexity of this riddle was indeed in the 3rd guard, who answers yes or no at random and tries to make it very hard for us. We need to get rid of him!
  3. The first question then tries to get rid of one of the guards and break down the riddle to the classic one with just 2 guards -one guard always tells the truth/ the other one always lies- in front of the 2 doors, one leading to Paradise and the other to Death. In the film “The labyrinth” we can also find a good example of the classic riddle.

How can we do that? Since there is not one question whose answer would indicate which guard is guarding which door, here are some ways to get rid of the guard that answers at random:

(Please note that we have called the 3 different guards G1, G2 and G3. T would be the one that tells the truth, F is the one that lies and T/F the one that answers randomly).

Do you trust G2 more than G3?
Would G2 be more likely to tell the truth than G3?
If the answer is YES, we would know that G3 is not the random one. If the answer is NO, we would know that G2 is not the random one.

 

Riddle_3doors_guards_table

Q1 = F => G2 ϵ {T,F}

Q2 = T => G3 ϵ {T,F}

 

  1. The second question is to find out where the iWatch is ,ie, Paradise. How then? Once you have determined one of the “non-random” guards, you have broken the riddle down to the classic one.

If I asked the other guard (in our case, the non-random one) which door the iWatch is behind, would he indicate door 1? If he says YES, take Door 2. If he says NO, take door 1.  

The winner

There are different ways to solve it, as you may probably know. Do no hesitate to add a comment and include yours!

Here is the winner from the Spark Summit 2017, with another great approach:

winner_riddle_3guards_2doors

Thank you very much to all participants! Hope to see you at next Spark Summit. With a new riddle to solve, of course.


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