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Can someone help me understand this maths riddle? (slrpnk.net)

submitted 10 months ago by [email protected] to c/asklemmy

19 comments fedilink hide all child comments

I just read Dr. No by Percival Everett. It contains a maths riddle that I cannot get my head around. I tried searching online but I couldn't find any answers.

Here's the riddle:

There are three sheepherders who come to a bridge controlled by a troll and his two sons. He demands of them thirty sheep before they can pass. Each shepherd cuts out ten sheep from his flock and they give them to the troll. Once they have crossed, the troll decides that he should only have asked for twenty-five. He sends his sons after the men with five sheep. The sons decide to keep one sheep each and give three back to the herders. They do. Now it is the case that each shepherd has paid only nine sheep. Nine times three is twenty-seven. The trolls kept two. Twenty-seven plus two is twenty-nine. Where is the missing sheep?

Can anyone help me understand?

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[–] [email protected] 4 points 10 months ago (1 children)

In net, the transaction is for 27 sheep, with 25 being kept by the troll father and 2 by the troll sons, so 25+2=9+9+9.

[–] [email protected] 8 points 10 months ago* (last edited 10 months ago) (2 children)

I think I get that, but I still don't understand who has the 'missing sheep' from the original 30. Is it the troll father or the shepherds?

EDIT: Hang on, I got it now. There's no missing sheep, the riddle tricks you into adding the wrong numbers together!

[–] ook_the_librarian 5 points 10 months ago* (last edited 10 months ago)

Exactly. More precisely, the wording is making the reader confuse assets and debts. Which side of the ledger does that sheep actually belong on?

[–] [email protected] 3 points 10 months ago

It's more like they're using the wrong sign for addition / using addition instead of subtraction to confuse you. The shepherds start with x sheep. They pay 30 and have x - 30. They should get 5 back, but they only get 3, so they have x - 30 + 3 = x - 27. If the sons had given them the last 2, they'd have x - 27 + 2 = x - 25 = x - 30 + 5.

If you want to view it from the trolls' side, you need only multiply everything with (-1).