Czy znasz odpowiedź? Poznaj rozwiązanie zagadki z 1. wydania biuletynu elektronicznego z lutego 2020 r.

The first question to ask a witness is:

Which of these other witnesses is more likely to tell the truth?

The answers have to be divided into three possible cases:

1. The question was addressed to the truthful witness who would point to the occasional liar.
2. The question was addressed to the notorious liar. The truthful answer would be to point to the "always honest" witness. But, as a notorious liar, he selects the occasional liar (as in case 1).
3. The "occasional liar" was asked. He points arbitrarily towards the liar or the truthful witness.

Looking at the answers, you see that the "occasional liar" is either the questioned witness or the one to whom the other witnesses point to. Meaning, the person who is not questioned or pointed to by the other witnesses cannot be the "occasional liar". The person always says the truth or the untruth. The police officer now asks a second, more complicated question to this particular witness:
 

Which escape route would the witness who does not occasionally lie indicate ?

The following two cases are possible:

1. The one to whom the second question is asked always tells the truth. If the other witness (who is not the occasional liar) is questioned, this person would always point the wrong way. So, to answer the question truthfully, the truthful witness also indicates the wrong escape route.
2. The question is addressed to the notorious liar. If the question is addressed to the witness who always says the truth, the liar would point to the escape route used by the thief. Since the question "which way would the other person point to" was posed to the liar, he would, of course, not indicate the route that the honest witness would point to. Thus, he would indicate the wrong escape route.

In both cases, the interviewees point out the incorrect escape route. Now, the police inspector simply has to choose the other path to successfully complete his investigation and catch the thief.

To solve this riddle, it is necessary to ensure that the first question eliminates the possibility that the second question is addressed to the "occasional liar". If you have identified a person who always behaves the same way (regardless of whether he always lies or always tells the truth), then you can solve the riddle with the help of an appropriate second question.