What rule do you mean? How to easily determine if a number is divisible by 8? Something else?
.
Assume: You can tell whether or not a 2-digit number is divisible by 8 just by looking at it.
If you can't, then learn the 8s column of a standard 12x12 times table.
.
If the number isn't whole, then the number is not evenly divisible by 8.
If the number doesn't end in an even number (0, 2, 4, 6, 8), then the number isn't divisible by 8.
In any positive or negative whole number greater than 100, look at the 100s place and the last two digits (the tens and ones places together).
If the hundreds place is even, check the last two digits. If the last two digits are divisible by 8, then the whole number is divisible by 8.
If the hundreds place is odd, check the last two digits. Subtract 4 from the last two digits. If that number is divisible by 8, then the whole number is divisible by 8.
Example: 5782075834905768
The 7 in the hundreds place is odd, so take the last two digits and subtract 4: 68 - 4 = 64. If that number (64) is divisible by 8, then the whole number (5782075834905768) is divisible by 8. 64 is divisible by 8, so the whole number is divisible by 8.
Example: 57840578345266
The 2 in the hundreds place is even, so take the last two digits (66). If that number (66) is divisible by 8, then the whole number (57840578345266) is divisible by 8. 66 is not divisible by 8, so the whole number is not divisible by 8.
Example: 57840578234057834295.6
This is not a whole number (an integer), so it is not divisible by 8.
Example: 5748905238905783405
This number does not end in an even number, so it is not divisible by 8.