sum up all digits of a number

There are ten digits from 1 to 9 and 0, with help of those digits we  can generate as many  numbers as we like.When we add the digits of any number we get another number, again adding digits of that number we get another number and at last a final digit of between 1 to 9.
Ex 2309876=2+3+0+9+8+7+6=35 , 3+5=8
Now ,  we do some operation like adding or subtractions  or multiplication in these numbers
We get a result and apply the above rule of adding digits for all , we note that in result also satisfies that.
------234 +438=672

• 2+3+4=9
• 4+3+8=15=1+5=6

now add 9+6=15=6

• The result is 672 =6+7+2=15=1+5=6

It means, our result is correct.
For testing accuracy of calculation we can apply this method.