A storm topples 4 of 49 trees. Which 10 survivors form five straight rows of four?
Henry Dudeney's landowner has a square plantation of 49 trees planted in neat rows and columns. A storm blows down 4 of them. He decides to fell all but ten of the trees left standing, choosing them so those ten form five straight rows with exactly four trees in every row. Which ten trees does he keep?
Reveal the answer
The trick is that the five rows aren't parallel: they're arranged so several of the surviving trees each lie on two rows at once, where lines cross. That's how five rows of four trees can be built from just ten trees instead of twenty. From Henry Dudeney's Amusements in Mathematics (1917), 'A Plantation Puzzle', a classic points-and-lines dissection problem.
— Henry Dudeney, Amusements in Mathematics — 1917, 'A Plantation Puzzle'
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