It is divisibility by 3 and divisibility by 5.
1, 2, 3, 5, 6, 10, 15, 30
To test divisibility for 20, you need to use the tests for divisibility by 4 and 5.The test for divisibility by 4 is that the last 2 digits of the number, given as a 2-digit number, are divisible by 4.Example for 4:We are testing the number 11042.42/4 = 10.5 which is not a whole number. Therefore 11042 is not divisible by 4.The test for divisibility by 5 is that the last digit of the number is either 5 or 0.
all even numbers
15 divided by 5 is 3 and 15 divided by 3 is 5
Every number has a test for divisibility. The issue is that the tests get more complicated as the divisor increases. For primes up to 50, see either of the attached links.
If a number is divisible by 3 and 5, it is divisible by 15.
If the number is also divisible by 2 and 3
A number is divisible by 15 if it is divisible by the prime factors of 15, that is, 5 and 3.
A number is divisible by 6 if the number is divisible by 2 AND 3.