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Here's a Cool way to check a Multiplication problems.

The Number 9 (again?!?)

    You can check math problems to make sure you have the right answer by using the numbers that add up to 9.   In our problem we multiplied 365 by 458.  After we got the answer, we x'ed out the numbers that add up to 9 like 3 and 6 in 365 and then 4 and 5 in 458. 

Then we add up what's left over across and keep adding the answer until we get a single-digit number.  Then we multiply those two numbers.  If your answer is correct, your one-digit answer will match the one-digit sum of your original solution.  Try it without x'ing out the numbers that add up to 9.  Did the solution change?

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