If asked to pick six numbers from this array, no two of which are in the
same row or column, you could proceed as follows:
1. Choose any number from the array, and then cross out all remaining
numbers from the row and column in which it is lies (the numbers
chosen in this example appear in bold enlarged print in Figures 2–4).
2. Choose a second number from the numbers not yet eliminated and
then strike out all remaining numbers from the row and column in
which it appears (see Figure 3).
3. Similarly choose three additional numbers (5, 46 and 42), thus arriving
at Figure 4.
4. When “choosing” the sixth and final number, there is hardly any choice.