You only need twelve
counters—six of one
colour, marked A, C, E, G, I, and K, and the other six marked B, D, F,
H, J, and L. You first place them on the diagram, as shown in the
illustration, and the puzzle is to get them into regular alphabetical
order, as follows:
The moves are made
by exchanges of opposite
colours standing on the same line.
Thus, G and J may exchange places,
or F and A, but you cannot exchange G and C, or F and D, because in one
case they are both white and in the other case both black.
bring about the required arrangement in seventeen