$n(n+1)/2$ coins are placed on an equilateral lattice with side $n$, such that all but one coin are showing heads.
At each step, one is allowed to choose two adjacent coins $A$ and $B$, and then flip all coins on the line $AB$ (and its extension).
Characterize all starting configuration such that it's always possible to get all tails.
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Bagus juga soalnya. :D Kuncinya tinjau tiga koin di ujung. Setiap langkah pasti membalik 0 atau 2 di antaranya.
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