What is the divisibility rule for 21?

Answer 1

For any practical purpose, it is easier to simply divide, instead of looking for fancy divisibility rules. However, you can apply the divisibility rules for 3 and for 7. This works because (a) their product is 21, and (b) these numbers are relatively prime.

Answer 2

A number is a multiple of 21 if the difference between twice the last digit and the rest of the number is a multiple of 21

105 is a multiple of 21 because 10 - 5 x 2 = 0 and 0 is a multiple of 21