shane at hathawaymix.org Fri Nov 20 13:13:44 MST 2009 Sasha
Pachev wrote: > I just came up with this while teaching my son
about numbers. Let's > see how quickly somebody figures this
out. > If you had a 64 bit number written down in hex, how could
you quickly > tell without the aid of any computational device,
including pencil or > paper if it is divisible by 15? The 9's
rule in decimal becomes the 15's rule in hexadecimal. Add the
digits together in hex, then add those digits again, until you
get a single digit. If the single digit is F, then it's
divisible by 15. > How about 17? Two-digit representations of
multiples of 17 are easy enough, but more digits aren't obvious.
I'd look to rows in Pascal's triangle for powers More
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