People have shared their bewilderment over a brainteaser which claims to reveal whether you have a high IQ.
The puzzle involves solving a school maths problem which ensures every pupil in a teacher’s class gets a well-deserved prize for their year’s efforts.
Shared on Reddit, the puzzle describes: “A teacher has an advance maths class containing 7 students. Every class for the whole year ends with the teacher putting a random number from 1-7 on their heads (they can have the same number as someone else in the class, everyone could get a 5 for example).
“The students are allowed to look at each others numbers, but not communicate. They then have to guess what number is on their head. As long as at least 1 student guesses correctly every class, they will be taken out for pizza at the end of the year.
“All the students love pizza, especially well earned pizza, so what strategy can they employ to guarantee they get it? Their guess is to be written down, and the teacher takes it from their desk.” While many tried in vain to crack the correct answer, some were pleading for hints to guide them towards the solution.
One user was puzzled, asking: “Are the students allowed to not guess? Would the other students see the prior students not guessing? ” Another added their confusion, saying: “I don’t understand how this is possible, the numbers are independent from each other and no communication is allowed. So looking at the other students’ numbers wouldn’t give you any information on your number.”
A third chimed in with a query, writing: “Would there be no other form of communication either? So could you say all students completely blind, deaf, no sense of time, nothing and only get the information of the other students number by Morse code tipping on their hand?”
One person offered a potential answer, saying: “I have a solution when they are able to see the others writing, not what they write in any way but in which order they are writing. Like if the sum of the 6 other numbers is even, student A starts writing, if not he waits etc.”
The original poster provided a clue, as they wrote: “With just 2 classmates this would be easy enough (and the numbers only 1 or 2).” They elaborated on the strategy, adding: “If student A writes down the number on student B’s head, and student B writes down the number not on student A’s head, one of them will correctly guess the number on their own head.”
A final user, sharing the correct answer, explained: “The nth student assumes that the sum of all the numbers is n modulo 7. Since he knows the numbers on everyone else’s head, he can work out his own number using that. Now if the sum is m modolu 7, then the mth student’s assumption was correct, so he managed to work out his own number correctly. Success!
“Note that: We have 7 students which is exactly enough to cover all possibilities mod 7. Everyone else’s assumption was wrong so they all had incorrect answers. That doesn’t matter since we only needed one person to have the right answer.”