The Mensa Town Council has just created the world's largest digital display in the town square.
Thirty-six digits were placed -- logically, of course -- in a huge 6x6 grid.
Last night *gasp* four of the digits were stolen.
6 4 7 8 3 7
8 2 5 1 5 6
3 - 8 6 4 8
8 6 5 3 7 6
5 4 7 - - 5
- 8 6 4 7 8
The owner of the display found your Digital Supply ad in the yellow pages and called for your help.
You arrive; and, after a moment's thought, you pull four digits from your bag and begin the restoration project.
Which four digits did you select?
Where did you place them?
Why?