Let $f: \mathrm{R} \rightarrow \mathrm{R}$ be a function which satisfies $f(\mathrm{x}+\mathrm{y})=f(\mathrm{x})+f(\mathrm{y}) \forall \mathrm{x}, \mathrm{y} \in \mathrm{R}$. If $f(1)=2$ and $\mathrm{g}(\mathrm{n})=\sum_{\mathrm{k}=1}^{(\mathrm{n})} \mathrm{f}(\mathrm{k}), \mathrm{n} \in \mathrm{N}$ then the value of n , for which $\mathrm{g}(\mathrm{n})=20$, is :