# Solution: The Six Frogs - Solved

(related to Problem: The Six Frogs)

Move the frogs in the following order: $2,$ $4,$ $6,$ $5,$ $3,$ $1$ (repeat these moves in the same order twice more), $2,$ $4,$ $6.$ This is a solution in twenty-one moves — the fewest possible.

If $n,$ the number of frogs, be even, we require $\frac{n^2+n}2$ moves, of which $\frac{n^2-n}2$ will be leaps and $n$ simple moves. If $n$ be odd, we shall need $\frac{n^2+3n}2-4$ moves, of which $\frac{n^2-n}2$ will be leaps and $2n-4$ simple moves.

In the even cases write, for the moves, all the even numbers in ascending order and the odd numbers in descending order. This series must be repeated $\frac n2$ times and followed by the even numbers in ascending order once only. Thus the solution for $14$ frogs will be $(2, 4, 6, 8, 10, 12, 14, 13, 11, 9, 7, 5, 3, 1)$ repeated $7$ times and followed by $2, 4, 6, 8, 10, 12, 14 = 105$ moves.

In the odd cases, write the even numbers in ascending order and the odd numbers in descending order, repeat this series $\frac {n-1}2$ times, follow with the even numbers in ascending order (omitting $n-1$), the odd numbers in descending order (omitting $1$), and conclude with all the numbers (odd and even) in their natural order (omitting $1$ and $n$). Thus for $11$ frogs: $(2, 4, 6, 8, 10, 11, 9, 7, 5, 3, 1)$ repeated $5$ times, $2, 4, 6, 8, 11, 9, 7, 5, 3,$ and $2, 3, 4, 5, 6, 7, 8, 9, 10 = 73$ moves.

This complete general solution is published here for the first time.

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### References

#### Project Gutenberg

1. Dudeney, H. E.: "Amusements in Mathematics", The Authors' Club, 1917

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