**Usage of digit sum:**

The number is divisible by 3, when its digit sum is divisible 3.

The number is divisible by 9, when its digit sum is divisible 9.

**Instructions:**

Digit sum is the sum of all digits of the integer. We write DS(n).

For example: DS(483) = 4 + 8 + 3 = 15.

How much is DS(981)?

DS(981) = ________.

How much is DS(981)?

DS(981) = **18**.

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Digit sum