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 | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |                       |     |     |  
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   |-|   | |   | |   | |   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
               |     |     |           |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |           |                 |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |     |           |           |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   | |   | |   |-|   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
                           |     |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   |-|   |-|   | |   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |                 |           | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   |-|   |-|   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+

Here's another way to visualize this puzzle that makes the strings more obvious.  Each "string" is simply 7 contiguous cells in the grid.   Instead of using the lines to connect the boxes, I removed the borders between them.  I re-drew the puzzle this way myself before solving it, because it was so much easier to see. 

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

For comparison, here is the sample 5x5 puzzle drawn this way.  Note that A,B,C appear in alphabetical order in each "string" (starting at one end or the other--it doesn't matter), and that every column and row contains each letter exactly once.

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

 

Edited on October 9, 2005, 6:04 am

Posted by Ken Haley on 2005-10-09 05:47:29