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Casey, who is 13, knows her age is not divisible by 3. Use what you know about numbers to explain why she is correct.
Thirteen is not divisible by 3 because if you add the digits in 13 you get 4 (1+3=4). To be divisible by three, the sum would need to be 3, 6, or 9, but it's 4 which means her age is not divisible by 3. Therefore, when you divide 13/3, you'll have a remainder.