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Well, one would think the full 20 would be needed for the first opposing sides (whether top and bottom, or left and right) and 18 would complete the interior of the other sides of the border, so (20 x 2) + (18 x 2) or 40 + 36 which = 76.  Our generic formula would then be (K x 2) + ((K-2) x 2) = border tiles.  If you need to simplify that, it could be 2K + 2k-4, or
4k-4 so for our original 20 x 20, it would be 4(20)-4 or 80 - 4 which is again, 76.


Some dis-ambiguity may be required:
This is assuming the 20 x 20 INCLUDES the border tiles upon completion.
If we're measuring by Ann placing a 20  x 20 center of white tiles, and THEN the border is added,
the formula becomes,
((K + 2) x 2 ) + (K+2-2) x 2  which simplifies to 2K + 4 + 2K, or 4K PLUS 4
Thanks so much for your explanation to this word problem. Yes I just needed to know where to plug in the right numbers!