ValleyBall1 wrote:

Good point mbaqst! I didn't think about that. Since 152 is even, I decided to start with 2 and go from there. Eventually I came up with my prime box. Since 190 ends in 0, I started with 10 and went from there.

Memorizing the divisibility number properties helps...

<b>Divisible by 2</b> if the number is even

<b>Divisible by 3</b> if the sum of the digits is divisible by 3

<b>Divisible by 4</b> if the number can be cut in half twice

<b>Divisible by 5</b> if the number ends with 0 or 5

<b>Divisible by 6</b> if the number is divisible by 2 & 3

<b>Divisible by 8</b> if the number can be cut in half three times

<b>Divisible by 9</b> if the sum of the digits is divisible by 9

Other than multiples of 7, I'm not aware of any other rules for 7. Anyone have shortcuts for 7?

To check if a number is divisible by 7, multiple the last digit by 2 and subtract it from the rest of the number. If the remaining number is divisible by 7, the number is too.

For example:

Is 3675 divisible by 7?

(1) Multiply 5 with 2. We get 10

(2) Subtract 10 from 367. We get 357.

(3) Is 357 divisible by 7? Yes, we get 51.

(4) So 3675 is divisible by 7. (3675/7 = 525)