What are fluids?

Three identical vessels are filled up to the same height with three different liquids $A, B$ and $C$ of densities $\rho_A, \rho_B$ and $\rho_C$ (where $\rho_A > \rho_B > \rho_C$). The pressure at the bottom of the vessels is:

In a $U-$tube experiment,a column $AB$ of water is balanced by a column $CD$ of oil,as shown in the figure. Then the relative density of oil is

$A$ $U$-tube having a horizontal arm of length $20 \ cm$ has a uniform cross-sectional area of $1 \ cm^2$. It is filled with water of volume $60 \ cc$. What volume of a liquid of density $4 \ g/cc$ should be poured from one side into the $U$-tube so that no water is left in the horizontal arm of the tube? (Answer in $cc$)

Two liquids of densities $\rho_1 = 1$ and $\rho_2 = 2$ are mixed in equal masses. What is the density of the resulting mixture?

In Guericke's experiment to show the effect of atmospheric pressure,two copper hemispheres were tightly fitted to each other to form a hollow sphere and the air from the sphere was pumped out to create a vacuum inside. If the radius of each hemisphere is $R$ and the atmospheric pressure is $p$,then the minimum force required (when the two hemispheres are pulled apart by the same force) to separate the hemispheres is