Primes/Factors

Question 1

What are the divisibility rules?

Answer 1

2: If a number is even (you can check just by looking at the last
digit), itís divisible by 2.
3: Add the digits of the number. If the result is a multiple of 3,
the number is, too.
4. If the last two digits are a multiple of 4, the number is a
multiple of 4. (e.g., the last two digits of 524 are 24,
which is a multiple of 4, so 524 is a multiple of four.)
5. If the last digit is 5 or 0, the number is a multiple of 5.
6. If a number is divisible by 2 and 3, itís divisible by 6.
7. Thereís no easy rule. To quickly determine whether a number
is divisible by 7, itís best to use the “nearest neighbor”
method I described in the Mental Math chapter.
8. No good rule. Just keep dividing by 2. If you can divide the
number by 2, then divide the result by 2, and the result is
still an even number, youíve got a multiple of 8.
9. Similar rule to that of 3: if the sum of the digits is a multiple
of 9, the number is a multiple of 9.