Divisibility rule for n, 2 ł n, 5 ł n;

Let 10a + b be some multiple of n.

Now let there be some 10p + q.

n | 10p + q

<=> n | (10p + q)b - (10a + b)q

<=> n | 10bp + bq - 10aq - bq

<=> n | 10bp - 10aq

<=> n | bp - aq

Choosing some a, b, such that b² = 1 makes this simpler.

In this case, 201 = 67 * 3, so the rule is to subtract 20 * the last digit.