The 25 cells below are connected so that are formed 5 strings of 5 cells. In each string you can read the letters A, B and C in this order (disregarding the empty cells in the string), and more, in the whole grid each letter appears in every row and in every column once, and only once.

             +---+ +---+ +---+ +---+ +---+
             | A |-|   | | B |-| C |-|   |
             +---+ +---+ +---+ +---+ +---+
               |           |
             +---+ +---+ +---+ +---+ +---+
             | B | | A |-|   | |   | | C | 
             +---+ +---+ +---+ +---+ +---+
               |                 |     |
             +---+ +---+ +---+ +---+ +---+
             |   |-| C | |   | | A | | B |
             +---+ +---+ +---+ +---+ +---+
                           |     |     | 
             +---+ +---+ +---+ +---+ +---+
             |   |-|   | | C |-| B | | A | 
             +---+ +---+ +---+ +---+ +---+
               |     |                 |
             +---+ +---+ +---+ +---+ +---+
             | C | | B |-| A | |   |-|   | 
             +---+ +---+ +---+ +---+ +---+

Fill the 7 x 7 grid below the same way (there are 7 strings, each one with 7 cells), using the letters A, B, C and D. One letter (D) is already placed.

 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   |-|   |-|   |-|   | |   |-| D | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |                       |     |     |  
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   |-|   | |   | |   | |   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
               |     |     |           |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |           |                 |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |     |           |           |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   | |   | |   |-|   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
                           |     |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   |-|   |-|   | |   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |                 |           | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   |-|   |-|   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+

(In reply to No Subject by Kyle Winking)

Kyle, I think that you didnīt understand the meaning of "string". There is no 3rd or 4th, etc...

A "string" is formed by 7 cells that are connected by the lines of diagram, and in each string, disregarding the empy cells, you must read ABCD in this order, starting from one end or from the other.

One string is, for example, the following 7 cells (see the connections in the diagram): first cell of the second row, first 5 cells of the first row and the fifth cell of the second row.

Another string: 6th and 7th cells of the 7th row, 7th and 6th cells of the 6th row, 6th and 7th cells of the 5th row, and 7th cell of the 4th row.

Clear now?

Posted by pcbouhid on 2005-10-07 01:01:21