1996 OIM Problems/Problem 5

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Problem

Three pieces $A$, $B$ and $C$ are located one at each vertex of an equilateral triangle of side $n$. The triangle has been divided into equilateral triangles with side 1, as shown in the figure for the case when $n=3$.

Ibe11 2.gif

Initially all the lines of the figure are painted blue. The pieces move along the lines, painting their path red, according to the following two rules:

i. First, $A$ moves, then $B$, then $C$, then $A$ and so on in turns. On each turn each checker travels exactly one side of a triangle from one end to the other.

ii. No piece can travel along one side of a triangle that is already painted red; but it can rest on a painted end, even if there is already another piece waiting there for its turn.

Show that for all integers $n>0$ it is possible to paint all the sides of the triangles red.

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe11.htm