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

What is wrong with goeshg's answer?

If his last string is ordered incorrectly, shouldn't Mandy's 3rd, 4th, 5th, and 6th strings all be incorrect as well?

Posted by Kyle Winking on 2005-10-07 00:05:26