Using a Manometer to Measure Gas Pressure PROBLEM

Question

If the gas inside the flask in the above exercise is cooled so that its pressure is reduced to a value of 715.7 torr what will be the height of the mercury in the open-end arm? (Hint: The sum of the heights in both arms must remain constant regardless of the change in pressure.)

 

I am so lost I do not know how to do this. Choices are: 

49.0 mm
95.6 mm
144.6 mm
120.1 mm

 

Thank u so much and your help will be much appreciated!

Answer 1

We need the atmospheric pressure.