Question: Dominoes. Can you arrange the 28 dominoes of an ordinary set in a closed loop, so that each matches with its neighbor in the usual way? Can you do so if all dominoes with a 6 on them are removed? Can you state a general theorem about dominoes with numbers 0,1 , .. . , n on them? (Hint: consider each domino as an edge of a graph with vertices labeled 0, 1, . . . ,n.)