probability density function over a continous variable > The probability density function of evaporation $E\,\textrm{mm/day}$ on any day during a year in a watershed is given by $$ f(E)= \begin{cases} 1/5 & 0\leq E\leq 5\\\ 0 & \text{otherwise}. \end{cases} $$ The probability that E lies in between $2$ and $4$ in the watershed is ____. I know procedure to find out the answer which is $0.4$ (using continuous variable), but I don't understand core concept of it. I can't visualize the graph for $f(E)$. can somebody help me to visualize what they have been asking, I don't want to understand mathematics as a formula.

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You can think of $ f(x)\times \delta x $ as being roughly the probability that the evaporation is within a small amount of $ x $, or in formulae

$ P(x-\tfrac{1}{2} \delta x \leq E \leq x + \tfrac{1}{2}\delta x) \bumpeq f(x) \times \delta x $