3. Of the numbers that end in the digit 9, one thing we can do is>this: if we 'cast out' that 9, and if the remaing digits are a

That is of very little help because it tells if the number is divisible by

>multiple of 3, then, the original number, N, is a composite. For

>example, the following numbers are composite: 1239, 3339, 50109.

3, so that rules out very few of the cases.