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

submitted 9 months ago by [email protected] to c/[email protected]

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] 19 points 9 months ago* (last edited 9 months ago) (1 children)

Shepherders	Troll	Sons
30
	30
	25	5
3	25	2

Everything is fine, up until this bit:

Twenty-seven plus two is twenty-nine.

The total amount given to the troll and sons were 27 sheep (25 and the sons kept 2).

Where it gets confused is saying "The trolls kept 2" as if this were 2 more sheep on top of the 27 sheep. This leads to you erroneously getting to 29 sheep somehow. The 2 sheep are part of the 27, you can't do this.

The 3 other sheep out of the original 30 are now with the shepherders after the sons came and returned them.

[–] [email protected] 2 points 9 months ago

To add to this, “Where is the missing sheep?” is an example of a leading question. The question is based on the assumption that there is a missing sheep, when in fact there isn’t, leaving you flustered as you try and reconcile that.

This assumption is emphasised by framing the erroneous 29 you’ve created (where you’ve added this random extra 2 sheep) against the original 30.

If you paired up the actual number (27) against 30 instead, you would have the total number of sheep given back to the shepherders (3).

All sheep are accounted for here, as long as you do your maths correctly.