You can surprise anyone by showing how quickly you can calculate whether one number is divisible by 3 or not.
Say 1235678. How you know if it is divisible by 3?
Add all the individual digits of the given number.
That is: (1+2+3+5+6+7+8)=32.
32 is not Divisible by 3. Hence, the given number is not divisible by 3.
If a number is divisible by 3, the sum of all of its individual digits will also be divisible by 3.
Now let's take this number: 17746404.
Add them up:
So 17746404 is divisible by 3.
Now it's time for you to surprise someone.
My friend Anshul pointed out the trick is also applicable for 9. Thanks to him.