Converting Pressure Units in Hydrodynamics

[qpzm77gg]

For Q4.7, why P_oil = P_Hg ?
For Q4.8, why P_H2O = P_Hg ?
P is pressure.
How can I know they are equal from the question?

 

[RTW69]

4.7: h_vel=v²/2g the units will be meters of diesel oil. That is not a standard unit to measure head. It is usually measured in meters of water or Hg. To make the conversion convert first to a dynamic pressure, in Pa. P_d=head*specific weight of water (_H20. Head in Hg=P_d/specific weight of Hg

4.8 Total Head=Pressure head+Velocity head+potential head + friction head

Convert total head in m of water to m of Hg as described above

Hint: some times you can neglect friction head (friction in piping)