Divisibility Rule for 21

If you divide a number by 21 and you get a whole number as an answer (quotient), then that number is divisible by 21.

Instead of simply dividing your number by 21 to see if you get a whole number, you can use what we call the Divisibility Rule for 21.

The Divisibility Rule for 21 is used to determine if a number is divisible by 21. You can also call it the test of divisibility for 21.

To determine if a number is divisible by 21, take the last digit and multiply it by 2. Then subtract that from the rest of the number. If the result is divisible by 21, then the number is divisible by 21.

For example, 525 is divisible by 21 because 52 - (5 x 2) = 42 and 42 is divisible by 21.

However, 526 is not divisible by 21 because 52 - (6 x 2) = 40, and 40 is not divisible by 21.

Divisibility Rule for 22
Here is the next Divisibility Rule on our list.

Divisibility Rules
Here is a list of all our Divisibility Rules.