Is this what you have in mind?
Code: 1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
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21 22 23 24 25 26 | 27 28 | 29 30
31 32 33 34 35 36 | 37 38 | 39 40
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41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80
81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 100
If so, call the upper left number (27) X. Then, the upper right number is X+1; the lower left number is X+10; and the lower right number is X+11.
The difference of the products is
(X+1)*(X+10) - X*(X+11)
What can you conclude from this?
What if the box was drawn around a different set of four numbers?