Alan has given a proof for the 7* one here
For seven:
Take the last digit, double it, and subtract it from the rest of the number. If the answer is divisible by 7 (including 0), then the number is also divisible by 7:
861:86-1x2=84
84/7=12
http://web2.0calc.com/questions/prime-factorization-of-624
Thanks Alan and thejamesmachine
I'll add this to out sticky notes Reference material under "Determining factors of a large number"
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