Three mirrors are arranged in the shape of an equilateral triangle, with their reflective surfaces pointing inwards. There is an infinitesimal gap at each vertex of the triangle through which a laser beam may pass.
Label the vertices $A$, $B$ and $C$. There are 2 ways in which a laser beam may enter vertex $C$, bounce off 11 surfaces, then exit through the same vertex: one way is shown below; the other is the reverse of that.
<imgclass="img-responsive center-block"alt="one way in which laser beam may enter vertex C, bounce off 11 surfaces and exit through the same vertex"src="https://cdn.freecodecamp.org/curriculum/project-euler/laserbeam.gif"style="background-color: white; padding: 10px;">