Question

What is the test of divisible of $18$?

Answer 1

A number that is divisible by $18$ must be divisible by both $2$ and $9$

The inverse is also true:

A number that is divisible by both $2$ and $9$ must be divisible by $18$

Therefore, we just have to test for both divisible by $2$ and $9$

$\bullet$ If a number is divisible by $2$, its last digit must be even

$\bullet$ If a number is divisible by $9$, the sum of all its digit must be a multiple of $9$

If a number passes both tests, then it will surely be divisible by $18$