Brain Teaser 2

The following problem just takes a little time. It doesn't require any math knowledge. Although, experience with problem solving strategies can help as always.

Have fun, and don't give up ! :) It helps, if you print the prison plan and use a pencil.

There is more than one solution.

One Hundred Forty Five Doors

A prisoner was thrown into a medieval dungeon with 145 doors. Nine, shown by black bars, are locked, but each one will open if before you reach it you pass through exactly 8 open doors. You don’t have to go through every open door but you do have to go through every cell and all 9 locked doors. If you enter a cell or go through a door a second time, the doors clang shut, trapping you.

The prisoner (in the lower right corner cell) had a drawing of the dungeon. He thought a long time before he set out. He went through all the locked doors and escaped through the last, upper left corner one.

What was his route?

cource: http://illuminations.nctm.org/BrainTeasers.aspx?id=5024

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