MathNet 2007-part-01 — 187 problems (EN–VI) Question 30

For $n = 1, 2, \ldots$ let $1 + \frac{1}{2} + \ldots + \frac{1}{n} = \frac{u}{v}$, where $u$ and $v$ are coprime positive integers. Find all $n$ for which $u$ is divisible by $5$.