Calculating squares on the outside of a grid

For the past week we have been learning about how to calculate the squares on the outside of a 10 by  10 grid we came up with 6 calculations on how to do it. Today I will share two with you, one I came up with and the other my class found out together.

First Method:

10 x 4 - 4 = 36

So the reason for this is there are 10 squares of each side of the grid and there are 4 sides. So we do 10 x 4 but then we takeaway 4 because the corners are counted more than once.

Now lets replace the numbers with letters,

4n - 4 =36
Lets try this theory on a larger grid, lets do a 60 by 60.
60 x 4 = 240, 240 - 4 = 236

Final Method:

10 + 10 + 8 + 8 = 36

So the reason for this one is we count the whole row on the top and bottom which each are 10 then take the corners away and count the two sides and you have two 8's.

Convert numbers to letters:

n + n + (n - 2) + (n - 2) = 36
Lets try this one on a larger grid, 60 + 60 + 58 + 58 = 236.

This is the grid with all the answers,

Do you think there are any other ways to work out this problem?

Let me know in the comments below.