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 multiplying 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 323490 instead of 233490 in the multiplication problem. Use Casting Out Elevens in order to catch digit reversal errors.
{% scaleimage l404_example.png %}
Find the digit sum for each number being multiplied.
Multiply each digit sum together.
Find the digit sum for the multiplication problems answer.
The multiplication problems answer is probably correct if the digit sums found in Step 3 and Step 4 are the same.
3 = 3 so 233490 is probably correct.