My grandson has a question If the computer multiplies 9x9 100 times, what is the ones digit of the product. What does the ones digit mean? Is that the last number in the ones column?

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Yes, that will be the last digit in the number. For instance, for 9x9=81, the ones digit is 1. For 9x9x9=729, the ones digit is a 9. For 9x9x9x9=6,561, the ones digit is a 1. That's my clue to your problem, too--notice any patten?