Is 642 divisible by 3

Answer 1

Yes because 6+4+2 is divisible by three.
The simple way to decide if any number is entirely divisible by three is to first discard any digit in the number that is divisible by three (3, 6, 9) then add any of the digits remaining together and if at any point you get something that is divisible by three you can disregard those digits. If at the end you have a remainder, the number is not evenly divisible by three. If there is no remainder the entire number is divisible by three.
In your example 642, I threw away the 6 leaving 4 and 2 , added those = 6 which is divisible by three (actually I remembered that 42 was divisible by 6 so I knew right after I had taken away the first 6 that your number was divisible by 3.