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.
LCM [9, 21] ==63
[10^63 - 1] / 9 mod 9 ==0
[10^63 - 1] / 9 mod 21 ==0
So, 63 reunits is the smallest number that will divide 9 and 21
111,111,111,111,111,111 / 9 = 12345679012345679 × 9 + 0
111,111,111,111,111,111/ 21 = 5291005291005291 × 21 + 0
