# Math Help - weird problem

1. ## weird problem

Hi, i didnt know where to put this problem

Show that every positive integer has a multiple made of only sevens and zeros.

2. So we have an integer $n$ and we want an integer $m$ such that $nm = s$, where $s$ consists of only 7's and 0's.

This is a constructive proof, but I'm sure there are other ways of doing it.

Firstly take a number of the form $x = 1111\ldots 111$, constisting only of 1's. Then calculate $r = x \mod{n}$

If $r=0$ then we can simply multiply r by 7 and we are done.

If not then we find 2 other numbers of the form $y_1=111\ldots 111$ > $y_2 = 11\ldots 111$ such that $y_1 \equiv y_2 \mod{n}$. Then $y_1 - y_2$ is divisible by n and consists only of 1's and 0's, we simply multiply this by 7 and we are done.

The justification for the existence of such $y_i$ is that if we take $n$ elements from the set $\left\{1,11,111,1111,\ldots \right\}$ , then there must be at least 2 with the same remainder mod n.

Pomp.