Mathematics - Functions Question with Solution | TestHub

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