proof again

OK, so i understand the first one, and got that and thanks for the
help, i got another one for ya.

Prove directly that for a positive integer written in base eight, say
n=(ak ak-1.....a2a1a0)8 = ak * 8K + a1*81 + a0* 80) (k k-1 2 1 0
and 8 should be sub scripts) we have that n is divisible by 7 if and
only if the sum of the digits ak + ak-1+....+a0 is divisible by 7.
Use this technique (rewrite in base 8 and sum the digits) to test
whether n=8981 is divisible by 7



any help on this one???

.

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