Question

How to find a sum of a number

Answer 1

you have to have two numbers to have a sum. in addition when you add two or more numbers the result you get is a sum.

Answer 2

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 .