The value $V$ of an asset after a certain period is given by the formula $V=P-\frac{P(1-r)}{k}$, where $P$ is the initial price, $r$ is the depreciation rate (as a decimal), and $k$ is a constant. If $V=1800$, $P=2000$, and $k=5$, what is the depreciation rate $r$?