Answer
0.039 ft/min
Work Step by Step
Volume V= Base area $\times $ Height
$=18\,ft^{2}\times h $
$\frac{dV}{dh}=\frac{d}{dh}(18\,ft^{2}\times h)= 18\,ft^{2}$
$\frac{dV}{dt}=0.7 ft^{3}/min $
$\frac{dh}{dt}=\frac{\frac{dV}{dt}}{\frac{dV}{dh}}=\frac{0.7\,ft^{3}/min}{18\,ft^{2}}= 0.039\,ft/min $
