Beach Day by Rodger Bliss
July 2024

Stone Bridge

Three people, Ann, Ben and Jen want to cross a river from left bank to right bank. Another three people, Tim, Jim and Kim want to cross the same river from right bank to left bank.

However, there is no boat and only one stone bridge consisting of just seven big stones (not tied to each other), each of which can hold only 1 person at a time. All these people have a limited jumping capacity, so that they can only jump to the stone immediately next to them if it is empty.

Now, all of these people are quite arrogant and so will never turn back once they have begun their journey. That is, they can only move forward in the direction of their destination. They are also quite selfish and will not help anybody travelling in the same direction as themselves.

But they are also practical and know that they will not be able to cross without helping each other. Each of them is willing to help a person coming from opposite direction so that they can get a path for their own journey ahead. With this help, a person can jump two stones at a time, such that if, say, Ann and Tim are occupying two adjacent stones and the stone next to Tim on the other side is empty, then Tim will help Ann in directly jumping to that stone, and vice versa.

Now initially the six people are lined up on the seven stones from left to right as follows:

Ann Ben Jen empty Tim Jim Kim

Your job is to find how they will cross over the stones such that they are finally lined up as follows:

Tim Jim Kim empty Ann Ben Jen

Now, find out the shortest step-wise procedure, assuming that Tim moves first.

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3 guesses to Stone Bridge

  • Kayla

    I got it in 21 moves:
    Firt Move: Ann Ben Jen Tim empty Jim Kim
    Second: Ann Ben empty Tim Jen Jim Kim
    Third: Ann Ben Tim empty Jen Jim Kim
    Fourth: Ann Ben Tim Jim Jen empty Kim
    Fifth: Ann Ben Tim Jim empty Jen Kim
    Sixth: Ann Ben Tim empty Jim Jen Kim
    Seventh: Ann empty Tim Ben Jim Jen Kim
    Eighth: Ann Tim empty Ben Jim Jen Kim
    Ninth: Ann Tim Jim Ben empty Jen Kim
    Tenth: Ann Tim Jim empty Ben Jen Kim
    Eleventh: Ann Tim empty Jim Ben Jen Kim
    Twelfth: empty Tim Ann Jim Ben Jen Kim
    Thirteenth: Tim empty Ann Jim Ben Jen Kim
    Fourteenth: Tim Jim Ann empty Ben Jen Kim
    Fifteenth: Tim Jim Ann Ben empty Jen Kim
    Sixteenth: Tim Jim Ann Ben Kim Jen empty
    Seventeenth: Tim Jim Ann Ben Kim empty Jen
    Eighteenth: Tim Jim Ann empty Kim Ben Jen
    Nineteenth: Tim Jim Ann Kim empty Ben Jen
    Twentieth: Tim Jim empty Kim Ann Ben Jen
    Twenty First:Tim Jim Kim empty Ann Ben Jen

  • Claire Lim

    I got mine in 15 moves :)
    01: Ann Ben Jen Tim emp Jim Kim
    02: Ann Ben emp Tim Jen Jim Kim
    03: Ann emp Ben Tim Jen Jim Kim
    04: Ann Tim Ben emp Jen Jim Kim
    05: Ann Tim Ben Jim Jen emp Kim
    06: Ann Tim Ben Jim Jen Kim emp
    07: Ann Tim Ben Jim emp Kim Jen
    08: Ann Tim emp Jim Ben Kim Jen
    09: emp Tim Ann Jim Ben Kim Jen
    10: Tim emp Ann Jim Ben Kim Jen
    11: Tim Jim Ann emp Ben Kim Jen
    12: Tim Jim Ann Kim Ben emp Jen
    13: Tim Jim Ann Kim emp Ben Jen
    14: Tim Jim emp Kim Ann Ben Jen
    15: Tim Jim Kim emp Ann Ben Jen

  • I got mixed up on this one because of the names haha. Good riddle though! If you dont mind, let me share this tricky fishing and boat riddle to see if anyone guesses it: Pete And Repeat