Henry Ernest Dudeney/Puzzles and Curious Problems/342 - A Wily Puzzle/Solution

Puzzles and Curious Problems by Henry Ernest Dudeney: $342$

A Wily Puzzle

A life prisoner appealed to the king for pardon.

Not being ready to favour the appeal, the king proposed a pardon on condition that the prisoner should start at cell $A$

and go in and out of each cell of the prison, coming back to the cell $A$ without going into any cell twice.

Solution

In the diagram it will be seen that the prisoner's course is undoubtedly all right until we get to $b$.

If we had been the prisoner, when we got to that point we should have placed one foot at $c$, in the neighbouring cell,

and have said, "As one foot has been in cell $c$ we have undoubtedly entered it,

and yet when we withdraw that foot into $b$ we do not enter $b$ a second time,

for the simple reason that we never left it since we first went in."

Sources

• 1932: Henry Ernest Dudeney: Puzzles and Curious Problems ... (previous) ... (next): Solutions: $342$. -- A Wily Puzzle
• 1968: Henry Ernest Dudeney: 536 Puzzles & Curious Problems ... (previous) ... (next): Answers: $434$. A Wily Puzzle