If you’re of a certain age, you may remember your school teachers telling you that you wouldn’t always have a calculator with you. Look how that one turned out. Regardless, math remains a core subject, and students often bring their homework to their parents to test what they can remember from decades ago.
One father from Lincolnshire, UK, was left despairing at one of his 7-year-old’s math questions. Mark told Newsweek “I support them having homework. As a child I hated it as all children probably do. But I see the value of it now. Although I did it before the days of group chats and AI and all that.” Despite dismissing a few modern learning aids, Mark enlisted the help of Reddit with a post titled “My 7 year olds homework is making me question my sanity.”
Posting under the name Kumquat_may, Mark explains, “I can’t see any mistakes. The number line appears to, indeed, show eighths.” His post has sparked hundreds of comments and over 1.4k likes in what one commenter identifies as ‘The Fencepost Error.’
An Ancient Math Problem
In the math question, Eva says, ‘The number line shows eighths,’ and the student must explain why she is incorrect. While there are indeed eight lines, there are only seven segments, so the number line shows sevenths. One user, TomT9 explains it best, “It should start at zero. It actually shows 7ths, imagine it as a loaf of bread and count how many slices there are.”
A reply with 4.8k likes by user ‘loveswimmingpools’ says, ‘The number line actually shows 7ths.’ VodkaMargarine elaborates, clarifying that problems like this have a name. “It’s called the fencepost error, and it’s been written about for thousands of years. The Romans needed to know about this so they could work out how many pillars to put around a temple,” they explain.
Another user, PuzzleHeadedLow4687, explains that it’s called the fencepost error because if you had eight fence panels, you would need nine posts to prop them up. Many other users reply with similar takes on the solution, although Kind-Mathematician18 hilariously adds, “Second mistake Eva has made was getting out of bed this morning. Plus exposing her ineptitude on Reddit wasn’t a smart move, either.