
Volume of water:
$V = \frac{1}{2}[ \, 2 + (2 + 2x) \, ]y (10)$
$V = 20y + 10xy$
From the figure:
$\dfrac{x}{y} = \dfrac{1}{2}$
$x = \frac{1}{2}y$
$V = 20y + 5y^2$
$\dfrac{dV}{dt} = 20\dfrac{dy}{dt} + 10y \dfrac{dy}{dt}$
when y = 6 in or 0.5 ft
$10 = 20\dfrac{dy}{dt} + 10(0.5) \dfrac{dy}{dt}$
$\dfrac{dy}{dt} = 0.4 \, \text{ ft/min}$ answer