Friday, 18 October 2013

727, A Prime, and Airplane

Ok, you knew 7 was prime....

And you could figure out pretty quickly that 727 is prime, probably.

What about 72,727; or 7,272,727,

or even 727,272,727 ???

One of those is divisible by seven...

Is there a quick rule for which of 72727..........2727 is prime?

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