Solution to CyberTeaser B178
As a coin rolls a distance equal to its circumference, it makes one full revolution. The perimeter of the rectangle is 12 circumferences. So the outside coin will make 12 revolutions as it rolls along the rectangles sides. In addition, at every vertex of the rectangle it makes an additional quarter turn (see the figure at right). So the total number of revolutions for the outside coin is 13.
The inside coin travels a distance 12c 8r, where c is its circumference and r = c/2(pi) is its radius. So it makes 12 4/(pi) [approximately 10.7] revolutions.
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