I study mathematics and first i tried to solve this problem using beatiful and complicated rules of dividing by 7 (i proved this rules myself;-)). But it was not a good way. So it is not task for mathematicians - this beatiful theorems are completly useless;-)
Now I've got AC...

You only need to read by simbol and use algorithm of long division.
I've got AC also =)

This link helps: http://ru.wikipedia.org/wiki/%D0%9F%D1%80%D0%B8%D0%B7%D0%BD%D0%B0%D0%BA_%D0%9F%D0%B0%D1%81%D0%BA%D0%B0%D0%BB%D1%8F
For those who don't read Russian, this is about  Pascal's criterion (http://en.wikipedia.org/wiki/Divisibility_rule)

@Katy, therefore, it is not true that theorems don't help in this problem :)

Edited by author 22.05.2013 19:14