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Crossing the Streams

This challenge is actually three challenges, but the same technique will help you solve all three:

  • Using fewer than 19 pieces, make the pattern brbrbrbr…
  • Using fewer than 21 pieces, make the pattern bbrbbrbbr…
  • Using fewer than 23 pieces, make the pattern bbrrbbrrbbrr…

If there’s anything we learned from Ghostbusters (1984), it’s that you shouldn’t cross the streams, except for the times that you should. The solutions presented in the back of the Puzzle Book for Challenges 8, 9, and 10 (shown below) are minimal, given the starting positions. However, if we allow the use of any pieces and any board configuration, these part counts can be reduced significantly. The trick is to use the same path for both red and blue balls, that is, to cross the streams. But wait, you might say, if we have red and blue balls on the same path, how will the machine be able to tell them apart? Well, that’s the question, isn’t it? :wink:

Good luck!



Rather than counting the red and blue balls separately, I count the them together, and use their position in the count to determine which side to deliver them to:

First ball goes to the red lever, second ball goes to the blue lever

First and third balls go to the blue lever, second ball goes to the red lever

First and fourth balls go to the blue lever, second and third balls go to the red lever

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Solution 1


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