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A repunit is a number consisting only of the digit 1, such as 111 and 11111. Find the number of digits in the smallest repunit divisible by 21 and 9.

 Sep 4, 2021
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[10^18 - 1] / 9 ==111,111,111,111,111,111 [18-digit number]

 

111111111111111111 mod 9 ==0

111111111111111111 mod 21==0

 Sep 5, 2021

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