If is a 3-digit number, then

is always divisible by

Let us collect numbers and their reversal.

Take for instance, , then its reversal is .

We know, in its general form can be written as,

And in its general form can be written as,

Taking their difference, we get

…(i)

Now, take . Expanding each in general form, we get

Then, we get

…(ii)

Adding equations (i) and (ii), we get

⇒ n = 18 (a – c)

We can easily say that, n is divisible by 18.