Answer
The water level is rising at a rate of $\frac{10}{3}~cm/min$
Work Step by Step
Let $b$ be the distance across the bottom of the trough. Then $b = 0.3~m$
We can use similar triangles to write an expression for the distance $B$ across the top of the water in terms of the water level $h$:
$B = 0.3+h$
We can differentiate both sides of the equation for the water volume with respect to $t$:
$V = \frac{h}{2} (B+b)~L$
$V = \frac{h}{2} (0.3+h+0.3)~(10)$
$V = 5h~ (h+0.6)$
$V = 5h^2+3h$
$\frac{dV}{dt} = (10h + 3)~\frac{dh}{dt}$
$\frac{dh}{dt} = (\frac{1}{10h+3})(\frac{dV}{dt})$
$\frac{dh}{dt} = [\frac{1}{(10)(0.3)+3}](0.2)$
$\frac{dh}{dt} = \frac{0.2}{6}~m/min$
$\frac{dh}{dt} = \frac{10}{3}~cm/min$
The water level is rising at a rate of $\frac{10}{3}~cm/min$
