- 13 Oct '07 19:42ok this will be very easy as i got it in just a few seconds myself but its fun to do nonetheless. find a number that is:

a number between 1 & 36

two digits

a multiple of 3

when the digits are added together the sum is between 4 & 8

is odd

when you multiply the digits the product is between 4 & 8

what is the number? ~~Dutch Defense~~Stealer of Souls- 13 Oct '07 23:56

15 r 17*Originally posted by tournymangr***ok this will be very easy as i got it in just a few seconds myself but its fun to do nonetheless. find a number that is:**

a number between 1 & 36

two digits

a multiple of 3

when the digits are added together the sum is between 4 & 8

is odd

when you multiply the digits the product is between 4 & 8

what is the number? - 14 Oct '07 11:51

Yup, it's a way to check if a number is a multiple of three;*Originally posted by joe shmo***Has anyone noticed the sum of the digits of multiples of 3 repeat 3,6,9 up until 39 which its sum is 12, which has a sum of 3 restarting the sequense replacing a sum of 12 fo three. can anyone explain this?**

Have a number abc (where a, b and c stand for the single digits of a three-digit number).

You can write it as a * 100 + b * 10 + c * 1

a * 99 is a multiple of 3, as well as b * 9.

So abc is a multiple of three

if and only if

a * 1 + b * 1 + c * 1 = a + b + c is a multiple of three.

This works for all numbers.

So to check if 123456723 is a multiple of three, we calculate 1+2+3+4+5+6+7+2+3 = 33.

To check if 33 is a multiple of three, we calculate 3+3 = 6

6 is a multiple of 3, so 123456723 is a multiple of three as well.

You have a similar trick for multiples of 9 (sum of digits must be 9 then) and multiples of 11 (alternating sum of digits must be 0).

There is even one for multiples of 7, but that's a bit trickier. - 14 Oct '07 14:32

Thanks, I find that interesting.*Originally posted by TheMaster37***Yup, it's a way to check if a number is a multiple of three;**

Have a number abc (where a, b and c stand for the single digits of a three-digit number).

You can write it as a * 100 + b * 10 + c * 1

a * 99 is a multiple of 3, as well as b * 9.

So abc is a multiple of three

if and only if

a * 1 + b * 1 + c * 1 = a + b + c is a multiple of ...[text shortened]... sum of digits must be 0).

There is even one for multiples of 7, but that's a bit trickier. - 17 Oct '07 20:59

or, you can just say that if the sum of digits is divisible by three, then the number is divisible by three...*Originally posted by TheMaster37***Yup, it's a way to check if a number is a multiple of three;**

Have a number abc (where a, b and c stand for the single digits of a three-digit number).

You can write it as a * 100 + b * 10 + c * 1

a * 99 is a multiple of 3, as well as b * 9.

So abc is a multiple of three

if and only if

a * 1 + b * 1 + c * 1 = a + b + c is a multiple of ...[text shortened]... sum of digits must be 0).

There is even one for multiples of 7, but that's a bit trickier.

you can do this multiple times if necessary...

for example:

111111111111111111111111111111111111111111111111111111111

sum of digits is 57 you can say that 57 is divisible by 3 since 5+7 = 12 and 1+2 = 3...

meanin that the entire number is divisible by three.