3 ms·

21=3*7 you are subtracting multiples of 21, then dividing by 10, which is coprime with 7

• orbitur 11y ago
  The examples subtracted 10, 16, etc... Those aren't multiples of 21.
  • chadzawistowski 11y ago
    Since you scratched off the last digit, 10 is really 100, plus 5 for the digit you scratched off = 105. 105 is a multiple of 21.
  • deleted 11y ago
    [deleted]