Question

A rectangular vessel is filled with water and oil in equal proportion, the oil being twice lighter than water. Show that the force on each side will be reduced by one fifth if the vessel is filled with only oil.

Answer 1

Answer is-
For rect. vessel containing water & oil both :
Pressure (Total) = ρwgx+ρogx 
And we are given with specific gravity of oil = ρo/ρw=1/2=0.5 . Thus, 2ρo=ρw 
Therefore, P (Total) = 2ρogx+ρogx=3ρogx 
For the vessel containing only oil : P (Total) = ρog(2x)=2ρogx 
Where : 2x being the height of the vessel.

Answer 2

The answer has been attached as a separate document to show that the force on each side will be reduced by one fifth if the vessel is filled with only oil.

There are in detail explanations available but this should serve your purpose.