For anyone who is wondering, I found out the answer. You assume atmospheric pressure pushes up into valve B, so the pressure in the tank is 14.7psi-(rho)gh where rho is the density of water, g is gravitational accel. (32.2ft/s^2), and h is 30ft. The point was that on the second part (h=40ft)...
I understand the concept of this question, I think, but I don't know how to go about getting an answer.
A tank is completely full of water with no air. A valve at the bottom is opened, while one on top remains closed. What is the lowest pressure that the tank will reach. From valve to valve...