On: Divisibility by 7 is a Walk on a Graph (2009)

[pervycreeper]

21=3*7

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

[raydev → pervycreeper]

The examples subtracted 10, 16, etc... Those aren't multiples of 21.

[chadzawistowski → raydev]

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.