in my theory you can calculate the outside (in red) 9+9+9+9 because it is a ten by ten grid and ten at top and ten at bottom so that leaves 8 and 8 at the sides because corners cant overlap (for e.g. you are allergic to fish and you were buying one for each friend in the red easy way is 10 times 4 but you have an extra 4 and you cant eat it so if you do 9 times 4 it would not count the corners twice.)
which shows to be the same answer as that 9+9+9+9 one they both add up to 36 and no overlapping corners will here's a question for you can you tell me other ways of finding out the red part of the 10 by 10 grid (without counting) ? Can you tell me if it worked on other squares (2 by 2 or 20 by 20)?
in another one of my theory's we could do 10+9+9+8 it works the same but has a harder way of working it out. because this is 10 plus (9 time 2) plus 8 which also equals 36