Eliminate any digits in the number that add up to 9 (including 9 itself).
Step 1
Remove any 9's and digits that add up to nine.
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Step 2
Add the remaining digits.
5 + 5 = 10
Step 3
Divide step 2 by 9. The remainder is the Casting Out Nines digit sum (can be zero).
10 / 9 = 1 Remainder 1
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