Limits Continuity And Differentiability

Suppose \(f: \mathbb{R} \rightarrow(0, \infty)\) be a differentiable function such that \(5 f(x+y)=f(x) \cdot f(y), \forall x, y \in \mathbb{R}\). If \(f(3)=320\), then \(\sum_\limits{n=0}^{5} f(n)\) is equal to :