**What is a digit?**

“Digit” is derived from Latin word *digitus* meaning ‘finger, toe’. Digit means any numeral below 10, i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.

And “Sum” means to add.

**A “Digit Sum” is the sum of all of the digits of a number, and is found by adding all of the digits of a number.**

For example:

- The Digit Sum of 32 is 3+2 = 5
- The Digit Sum of 152 is 1+5+2 = 8

*If the sum of the digits is greater than 9, then sum the digits of the result again until the result is less than 10.*

For example:

- The Digit Sum of 57 is 5+7 = 12 → 1+2 = 3 ….. Here 12 >9, so sum the digits again.

So the Digit Sum of 57 is 3.
- The Digit Sum of 687 is 6+8+7 = 21 → 2+1 = 3…. Here 21 >9, so sum the digits again.

So the Digit Sum of 687 is 3.

**Keep finding the Digit Sum of the result until it’s less than 10. Also, in case of digit sum, 0 and 9 are equivalent!**

Why is digit sum important?

Digit sum is used for checking calculations, and this becomes highly useful where large numbers are involved. I will explain this part in subsequent sections.

In this course, we are going to study about below topics:

- Nine Point Circle
- Casting Out nines (9s)
- Checking with Digit Sums
- A magic number

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