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*