Math Tutorial - 01: Concept of Digit Sum

What is Digit Sum?

For a given number digit sum is the sum of all its digits. If the sum is greater than 9 then its digits are again added to get the new sum until its sum lies between 1 and 9 i.e. a single digit number.

Examples:

Number	Digit Sum
34	3+4=7
123	1+2+3=6
7468	7+4+6+8=25,  2+5=7
332173	3+3+2+1+7+3=19,   1+9=10,   1+0=1
123456789	1+2+3+4+5+6+7+8+9=45,    4+5=9