People with 'high IQs' can solve 'impossible' brainteaser from 15 years ago
While some individuals can crack brainteasers in a matter of seconds, others need to sit down and repeatedly read the puzzle to try to solve it.
However, a few people are celebrating after finding the solution to a brainteaser that was first posted on Reddit 15 years ago. The brainteaser has been reposted in recent days as social media users attempt to solve it using only "logic and reasoning".
Originally posted on the AskReddit thread, it begins: "A monastery has 100 monks, all of whom have taken vows of silence. This means absolutely no communication no notes, gestures or anything like that. The only exception is the head priest who delivers a sermon every day.
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"During one such sermon, he announces that he has placed a mark upon the foreheads of some of the monks while they slept at night. He further says the day a particular monk figures out that he has a mark, he should stop coming to the daily sermons. The day after, all the monks are still there, as well as the day after that. Finally, on the fifteenth day, precisely those monks who bear the mark are absent."
Initially, players are asked to answer the question: "How many monks were absent?" before shifting their focus to the more crucial question: "How did they figure this out?"
Adding some crucial details, the puzzle goes on to say: "The monks have no access to anything in which they may see their reflection. In other words, they absolutely cannot see their own foreheads nor receive any indication from any other monk. Also, the mark doesn't fade or wear off during their ablutions or with the passage of time."
In the comments section, answers to the question ranged from 14 to 100 monks. However, others confessed they had "no idea" where to begin to solve the question.
Discussing the brainteaser, the poster said:" It is meant to test the thinking process, and how well you respond to something you can't possibly solve. More importantly, it tests your response to failure."
Solution
Revealing the correct answer, one user stated: "15 monks are marked. For answer two, there are rules to remember. A. Every monk knows at least one, probably more, monks have a mark at their head in the group. B. Every monk is not sure whether he has a mark.
"Deduction steps: Step 1. Consider the simple case. Only one monk has a mark, M1. M1 sees the rest are clear; applying the above rules, he realises he's the one with the mark; leaves on first day.
"Step 2. Consider the scenario where two individuals, M1 and M2, have marks. On the first day, M1 notices that M2 has a mark and the rest do not, but he can't confirm whether he himself has a mark, so he remains silent. The same goes for M2. The others see both of them with marks, but they can't be certain if they themselves have it, so they also remain silent.
"On the second day, M1 observes that M2 didn't depart after the first day's sermon. M1 then deduces that M2 must have seen another person with a mark. Since M1 sees that the rest don't have a mark, he must be the one with the mark that M2 saw in the first sermon. Therefore, he leaves on the second day. The same applies to M2.
"Step 3. And so on for the case of N person with mark. All the N persons with mark will realise they are the one after N days, and leave. Since the question states that after 15 days some people leave, it then follows that there must be 15 persons with mark on their heads and their realise they are the ones after 15 sermons."
