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If there’s one thing that brings everyone together (even if it’s only in frustration), it’s a good riddle. No matter what the subject, riddles and brainteasers make us think, and may even spark a healthy debate or two. And a math riddle was posted on the online forum Mumsnet that’s got a lot of people scratching their heads. Warning: You may wanna grab a pen and paper.

The riddle was originally posted by a user named PeerieBreeks. “Can’t understand how so many people on Facebook have it so completely wrong (and can justify it to themselves),” the user wrote. “Without adding your reasoning, tell me what you think the answer is.”

Here’s the riddle in question:

Over 500 responses came in on Mumsnet. And while many got it right, a lot of people had a hard time with it.

The answer is…the man profited by $20. Remember how we noted the pen and paper? First you need to look at the initial transaction, and see that he made $10. Then, separate out the second — by buying for $80 but then selling for $90, he made another ten dollars. The part that trips people up is when he sells for $70 and then buys back for $80, because it kind of makes it seem like he lost money.

One user named DadDadDad said it was much easier if you mentally switched up animals.

Who knew something so simple could still be so confusing?