The number $a_n$ is formed by writing in succession, without spaces, the numbers $1, 2, ..., n$ (for example, $a_{11} = 1234567891011$). Find the smallest number t such that $11 | a_t$.
Problem
Source: Czech-Polish-Slovak Junior Match 2014, Individual p4 CPSJ
Tags: number theory, divides, divisible, Digits