There is an 8 inch water main providing water to a 50 story building with a operating pressure of 60 PSI. What is the static pressure on a fixture 40 feet above the water main. Basically, the question is asking what is the water pressure on the fixture on the 40th floor? This is a two part problem but also rather simple. First you will take the 40 feet above the water main and multiply it by .433 and you get 17.32 psi. Next, you take the 60 psi operating pressure, the main operating pressure, and subtract 17.32 psi.
40 feet x .433=17.32 psi
60 psi-17.32 psi=42.68 psi, the operation pressure on a fixture 40 feet above the water main.