Answer
The depth of the water surface in the well is $~~12.4~m$
Work Step by Step
We can find the speed of sound in the well:
$v = \sqrt{\frac{B}{\rho}}$
$v = \sqrt{\frac{1.33\times 10^5~Pa}{1.10~kg/m^3}}$
$v = 347.7~m/s$
We can find the depth $L$ of the water surface in the well:
$f = \frac{nv}{4L}$
$L = \frac{nv}{4f}$
$L = \frac{(1)(347.7~m/s)}{(4)(7.00~Hz)}$
$L = 12.4~m$
The depth of the water surface in the well is $~~12.4~m$
