Tricky card brainteaser sparks arguments as people struggle to solve it

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A brainteaser involving playing cards has sparked heated debates online as people scratch their heads over the correct answer.

Posted on Reddit, the challenge begins: "You take a large pile of cards face down, flip 99 of them face up, and blindfold yourself. You must now shuffle them so you cannot tell which 99 are face-up, then produce two piles with an equal number of cards face-up. No damaging nor inspecting cards at all, only flipping and sliding across the table."

It continues: "There will be exactly 99 face up to start, so at least one flip is necessary." The puzzle requires a solution that works no matter how many face-down cards there are, stating: "It should handle 99 through 9999 cards, etc. Empty piles are fine, they have 0 face up." Users have been chiming in with their attempts to crack the conundrum.

One user confessed: "I am proud to say that I figured out exactly zero of these without help." Another suggested: "Flip them all, then flip a single random one." A different user proposed: "Separate them into one pile of 99 and another of the rest, and then flip the pile of 99." Meanwhile, another added: "If I am blindfolded, I would think they are all face down because you said you 'secretly' flipped 99 of them face up. I have no clue how to answer this but this is what is confusing me."

One user, after pondering the question for a while, finally came up with the correct answer: "Think I've got it, assuming I'm informed of the number of cards you flip... Divide into a pile of 99 and a pile of the rest. Flip the pile of 99. It should always be the same regardless of the specific numbers. Divide into a pile equal to the number of face-up cards, and then the remainder. Flip the pile equal to the number of face-up cards."

"Lets say 500 cards, 20 face up. Pile of 480 and 20. You know for sure there's between 0 and 20 face up cards in the pile of 20, and the remaining cards from the set of 20 in the other pile. If all 20 are in the stack of 480, then you create a matching face up stack of 20 when you flip it. If 19 are, then you have one face-up card in the stack of 20, and will match the 19 face up after the flip. And so on."

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