(b) Let $k$ be the force constant of spring of length$l_{2}$. Since $l_{1} = n\ l_{2}$ , where $n$ is an integer, so the spring is made of $(n + 1)$ equal parts in length each of length $l_{2}$
$\therefore\ \frac{1}{k} = \frac{(n + 1)}{k}\ \ \ or\ \ \ k = (n + 1)k$
The spring of length $l_{1}( = n\ l_{2})$ will be equivalent to $n$ springs connected in series where spring constant $k^{'} = \frac{k}{n} = \frac{(n + 1)k}{n}$