# Recurrence formula

Algebra Level 2

If $$f(n) : \mathbb{N}^+ \rightarrow \mathbb{N}^+$$ is a function that satisfies

$f(5)=4, f(5n)=f(n)+3,$

what is the value of $$\displaystyle \sum_{k=1}^{10} f(5^k)?$$

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