If $t_1$,$t_2$,... are natural numbers (none of them is $1$). Prove, that all $N \in \mathbb N$ can be written down (only in one way) in this form:
$$N=a_n\cdot t_n\cdot t_{n-1}...\cdot t_1+a_{n-1}\cdot t_{n-1}\cdot t_{n-2}\cdot ...\cdot t_1+...+a_1\cdot t_1+a_0$$
where $0\le a_i < t_{i+1}$ and $a_n$ is not $0$.
$$N=a_0+t_1 (a_1+t_2 (a_2 + t3 ( a_3 + .... t_n(a_n) ....)))$$
Division Theorem!!