Karyoplasma , 10 months ago

Take the last digit of the number, double it and subtract it from the rest. If that new number is divisible by 7, the original one is as well. For your example:

2794 - 6 = 2788

I know 2800 is divisible by seven, so 2788 is not. Thus 27943 is not divisible by 7.

Quick maff shows that neither subtracting 3 or adding 4 will make the original number divisible by 7. Adding 1 or subtracting 6 will tho.