The sum of digits, also termed arithmetically as digit sum, is simply a sum of all the digits constituting a number. The only catch is that if the sum of digits constituting a number is a number that is greater than nine, then the digits of this number are added over and over until a number equal to or less than nine is obtained. The single-digit sum obtained after repeatedly adding the digits is called the digital root or repeated digital sum.
Instructions
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1
Take a number for which the digit sum is to be determined. Let's suppose that the number is 666.
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2
Add the digits of the number. In this case, adding 6 + 6 + 6 gives you 18, which is a number greater than nine.
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Add the digits of the number again until you get a number that is equal to or less than nine. In this case, adding 1 + 8 gives you 9. As such, the digit sum for the number 666 is 9.