The probability density function of evaporation $E$ on any day during a year in a watershed is given by

$f(E) = \begin{cases} \dfrac{1}{5} & 0 \leq E \leq 5\: \text{mm/day} \\ 0 & \text{otherwise} \end{cases}$

The probability that $E$ lies in between $2$ and $4$ mm/day in a day in the watershed is (in decimal) ____________