A geometric sequence has first term $a$ and common ratio $r$, where $a$ and $r$ are positive integers and $r$ is greater than 1.
The sum of the first $n$ terms of this sequence is denoted by $S _ { n }$
It is given that the terms of the sequence satisfy
$$S _ { 30 } - S _ { 20 } = k S _ { 10 }$$
for some positive integer $k$.
What is the smallest possible value of $k$ ?