Casting Out Nines is a very fast way to sanity check your work. It is a great way to quickly catch most errors. Casting Out Nines can be used when adding numbers of any size.
It is important to note that Casting Out Nines can tell you if an answer is definitely incorrect. Failing the Casting Out Nines test means that an answer is incorrect. However, passing the Casting Out Nines test still leaves a small potential that an answer can still be incorrect. Therefore, Casting Out Nines can tell you if an answer is wrong but it cannot confirm that it is right.
Casting Out Nines will not catch an error in digit reversal. For example, finding the answer as 570584 instead of 750584 in the addition problem. Use Casting Out Elevens in order to catch digit reversal errors.
Example
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Step 1
Find the digit sum for each number being added.
424238 digit sum is 5
326346 digit sum is 6
Step 2
Add together each digit sum.
5 + 6 = 11
Step 3
Find the digit sum of the answer in Step 2
11 digit sum is 2
Step 4
Find the digit sum for the addition problems answer.
750584 digit sum is 2
Step 5
The addition problems answer is probably correct if the digit sums found in Step 3 and Step 4 are the same.
2 = 2 so 750584 is probably correct.
Bonus
Casting Out Nines can also be used for checking your work when adding multiple numbers together.
Example
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424238 digit sum is 5
326346 digit sum is 6
948724 digit sum is 7
650243 digit sum is 2
5 + 6 + 7 + 2 = 20
20 digit sum is 2
2349551 digit sum is 2
2 = 2 so 2349551 is probably correct.