A hollow pressurized sphere having radius r = 4.8 in. and wall thickness t = 0.4 in.

Question:

A hollow pressurized sphere having radius r = 4.8 in. and wall thickness t = 0.4 in. is lowered into a lake (see figure). The compressed air in the tank is at a pressure of 24 psi (gage pressure when the tank is out of the water).
At what depth D0 will the wall of the tank be subjected to a compressive stress of 90 psi?