Answer
27 meters
Work Step by Step
We have to find the ratios between the two heights, which we will call $h_1$ and $h_2$. Thus, we find:
$\frac{h_2A\rho_wg}{h_1A\rho_wg}=\frac{3mg}{mg}$
Thus, we see that the ratio is 3 to 1. This means that the minimum depth is:
$h= 9(3)=27m$
