divisibility rule for 21

A number is divisible by 21 if it is divisible by both 3 and 7. Here are the steps to check:

1. Divisibility rule for 3: Add up all the digits in the number. If the sum is divisible by 3, then the number is divisible by 3. For example, consider the number 126. The sum of the digits is 1+2+6=9, which is divisible by 3. Therefore, 126 is divisible by 3.

2. Divisibility rule for 7: Double the last digit and subtract it from the rest of the number, if the result is 0 or a number that is divisible by 7, then the number is divisible by 7. For example, consider the number 126. Double the last digit (2*6=12) and subtract it from the rest of the number (12-12=0). Since the result is 0, 126 is divisible by 7.

If the number passes both these tests, then it is divisible by 21.