How to find a sum of a number

There's no such thing as the sum of one number.  "Sum" is what you have
when you moosh two numbers together into one by adding them.

For example:

If you put 5 pennies into your pocket, and then later, you put 3 more pennies
into there, what's in your pocket now ?  Don't look, do it in your head. 
You said there are 8 pennies now. That's correct. How did you figure it out ?
You 'added'  5  and  3  and you got  8 .    8  is the 'sum' of  5  and  3 .

Since the area of the floor is 100 and one side is 10, then we should know that the other side is 10 because, 
10 * 10 = 100

So, since two sides are 10, than the other sides must be 10 too.

That gives 4 sides that all equal 10.

10+10+10+10= 40

So, the perimeter is 40.