D18. Domino Design

Contributed by Ian Stewart, University of Warwick

Figure 1: A row of dominoes with five pips at the ends and five in the middle.

In the arrangement of five dominoes shown in Figure 1, obeying the usual rule that adjacent squares have matching numbers, the two at the end total five spots, and so do the three in the middle. There are three other ways to achieve this, not counting left-right reflections as different. Can you find them?

This problem originally appeared in the Prisoner’s Dilemma in the 2024 Spring issue of the PMP Newsletter. Solutions will be accepted through 2024 May 15.