Five would be 4 and 1... since those are numbers that can be represented by a binary "marble" in the right position, and each of those positions has a slot in the machine.
So you put a marble into 4 and 1 slots, and let them drop to store a "5" in the register. Then you put another into the 4 slot and let it drop, it clears the marble already in 4 through the drop-chute, flipping the flip-flop and pushing the new marble up from 4 slot to the 8, and you very simply get 9 which is 8 and 1.
So you put a marble into 4 and 1 slots, and let them drop to store a "5" in the register. Then you put another into the 4 slot and let it drop, it clears the marble already in 4 through the drop-chute, flipping the flip-flop and pushing the new marble up from 4 slot to the 8, and you very simply get 9 which is 8 and 1.