The left arm of a manometer is connected to a container containing gas ‘X’ and the mercury level in the right arm is raised by 2 cm. Now without disconnecting the container of gas X, the right arm of the manometer is connected to another container containing gas ‘Y’ and the mercury level in the right arm is pushed down by 5 cm. Find the pressure exerted by the gases ‘X’ and ‘Y’.

When the left arm of the manometer is connected to the container of gas X, the mercury in the right arm is raised by 2cm.
The mercury in the left arm is pushed down by 2cm, as a result the difference in the mercury level in both the arms is 4cm.
Now , the pressure of gas X= Atmospheric pressure+ Pressure due to a column of 4cm of mercury .  
pressure of gas X=76cm  of Hg +4cm of Hg
Pressure of gas=80cm of Hg
When the right arm is connected to the container of gas Y , the mercury arm is pushed down by 5cm.
Hence , the difference in the levels of mercury columns in both the arms = 5cm−2cm)×2⇒6cm
Thus , the pressure of gas Y= pressure of gas X+ Pressure due to a column of 6cm
of mercury . 
pressure of gas Y=80cm of Hg + 6cm of Hg
Pressure of gas Y=86cm of Hg .