Thanks to Daniel E for sharing this puzzle! Also do check out these sources for the historical background to this problem and other similar “killer problems” that are easy to state and nearly impossible to solve unless you know the trick (in which case they are readily solvable problems).
My blog post for this video:
Sources and historical background
Gold Plated Goof video on “coffin problems” – Simple Math Problems To Fool The Best. The triangle problem is the third problem presented (around 5:33)
Tanya Khovanova, and Alexey Radul. “Jewish Problems.”
Originally published as: Tanya Khovanova, and Alexey Radul. “Killer Problems.” The American Mathematical Monthly, vol. 119, no. 10, 2012, pp. 815–823. JSTOR, JSTOR, www.jstor.org/stable/10.4169/amer.math.monthly.119.10.815.
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