Challenge

[Dave Smith]

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?
>
> How about 17?

For 15: If you add up the numeric value of each digit, it should total 
15. Did your son come up with an answer? If so, great job!

--Dave