What I use for 11 is take the final digit, subtract the penultimate, add the third last, etc. The result = the original number (mod 11). Obviously it works if y

What I use for 11 is take the final digit, subtract the penultimate, add the third last, etc. The result = the original number (mod 11). Obviously it works if you start at the MSD, but then you need to worry about whether there's an even number of digits, in which case you need to negative the answer. Seven wasn't new to me, but I've never been able to remember it. Maybe I will this time.