The 22 dots in the figure below can be connected by 11 lines, each beginning and ending with a dot (o) and passing through 4 empty cells. Connecting lines may proceed horizontally or vertically, but not diagonally. They may change direction as often as needed, and they should not cross one another.

               +---+---+---+---+---+---+---+---+
               |   | o | o |   |   | o |   | o |
           +---+---+---+---+---+---+---+---+---+---+
           |   |   | o | o | o |   |   |   |   |   |
           +---+---+---+---+---+---+---+---+---+---+
           |   |   |   |   |   | o |   |   |   | o |
           +---+---+---+---+---+---+---+---+---+---+
           | o |   |   |   |   |   |   | o |   | o |
           +---+---+---+---+---+---+---+---+---+---+
           | o |   |   | o |   |   | o | o |   |   |
           +---+---+---+---+---+---+---+---+---+---+
           | o |   |   | o |   | o |   |   |   |   |
           +---+---+---+---+---+---+---+---+---+---+
               |   |   | o |   |   |   | o | o | 
               +---+---+---+---+---+---+---+---+

Can you do it?

(In reply to Solution by Lisa)

Nice solution Lisa.

Posted by Hugo on 2005-08-15 21:09:21