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How to do the sixth question on page 1 1 in the mathematics textbook of Chinese school (grade five)? (through practice)
My book is different from yours. I don't know if this is the topic. But it seems to have nothing to do with vertical position.

6. Prove that if any three-digit number is written twice in a row to get a six-digit number, it must be divisible by 7, 1 1 and 13 simultaneously.

Let any three numbers be ABC, then six numbers.

ABCABC= 1000×ABC+ABC

= 100 1×ABC

=7× 1 1× 13×ABC

So this six-digit number must be divisible by 7, 1 1 and 13 at the same time.

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