ABSTRACT
To exchange their positions, knights should perform nine moves as follows: 18-13, 7-18, 13-6-7, 15-16, 12-9, 4-15, 1-12, 16-1, 9-4.
Each shape in the chain has the number of vertices indicated by the number next to it in the illustration on the left. When the chain is completed with the missing shape and then unfolded, as shown on the right, each indicated pair of shapes has nine vertices total. So, in order to complete the chain observing this rule, the missing solid should be double cone B, since it is the only solid with two vertices.
