The Mercury Manometer Shown In The Figure Is Attached To A Gas Cell Finding Density Using Ideal Gas Law?

Finding density using ideal gas law? - the mercury manometer shown in the figure is attached to a gas cell

The mercury manometer is shown in the figure, placed a gas cell. The height is 120 mm of mercury, when the cell is placed in a water-ice. The amount of mercury up to 30 mm when the device made in industrial freezers.

http://session.masteringphysics.com/problemAsset/1073972/3/knight_Figure_16_50.jpg

I know, I have the first and last pressure to find the ideal gas law to be. However, I have tried to find a solution and take the Patmos + p = density of mercury (region) at 0.12 m, but in which he said quesiton 120 mm ... Who knows why it is used 0.12 m? It is the only thing that screwed me this question!