Question

How to identify wether a number is divisible by 7 or not?

Answer 1

Remove the last digit, double it, subtract it from the truncated original number and continue doing this until only one digit remains. If this is 0 or 7, then the original number isdivisible by 7. Example: 1603 -> 160-2(3)=154 -> 15-2(4)=7, so 1603 isdivisible by 7.