What are the divisibility rules from 1-20?

Answer 1

21

Answer 2

1: Every number is a multiple of 1

2: The number ends in 0, 2, 4, 6 or 8

3: The sum of the digits is a multiple of 3

4: The last 2 digits are a multiple of 4

• The 10s digit is even and the last digit is 0, 4 or 8
• The 10s digit is odd and the last digit is 2 or 6

5: The number ends in 0 or 5

6: The number is a multiple of 2 and 3 at the same time

7: The difference between twice the last digit and the rest of the number is a multiple of 7

8: The last 3 digits are a multiple of 8

• The 100s digit is even and the last 2 digits are a multiple of 8
• The 100s digit is odd and the last 2 digits are 4 times an odd number

9: The sum of the digits is a multiple of 9

10: The number ends in 0

11: The difference between the last digit and the rest of the number is a multiple of 11

12: The number is a multiple of 3 and 4 at the same time

13: The sum of 4 times the last digit and the rest of the number is a multiple of 13

14: The number is a multiple of 2 and 7 at the same time

15: The number is a multiple of 3 and 5 at the same time

16: The last 4 digits are a multiple of 16

• The 1,000s digit is even and the last 3 digits are a multiple of 16
• The 1,000s digit is odd and the last 3 digits are 8 times an odd number

17: The difference between 5 times the last digit and the rest of the number is a multiple of 17

18: The number is a multiple of 2 and 9 at the same time

19: The sum of twice the last digit and the rest of the number is a multiple of 19

20: The number ends in 00, 20, 40, 60 or 80

Answer 3

For most cases, just doing the division is simpler than finding some special rule.