Divisibility by 7

7 is a prime number, but how do you know a number is divisible by that number?

Is this number divisible by 7?
If a number is divisible by 7, here are the steps: You can also do this another way. Its more complicated. take the number and group the digits into 6 groups. label them abcdefabcd...abcdef. Make sure it ends in f. Add all of the a's together, then multiply by 5. add the b's, multiply by 4. add the c's multiply by 6. Add the d's multiply by 2. add the e's, multiply by 3. add the a's. When you add the 6 results, you get a multiple of 7.
 * 1) Multiply the last digit by 2 (e.g. NNNN = NNN(2N))
 * 2) Subtract that number by the remaining number (e.g. NNN-(2N)=NNN)
 * 3) Repeat the process (e.g. NN-(2N)=NN, N-(2N)=N)
 * 4) Check the result. If it is divisible by 7, so is the original number, hence if N is divisible by 7, so as NNNN in the example.