A closed tank contains a vapor-liquid mixture of steam at 45 bar with liquid content 25% by mass.

a) The pressure is changed to 80 bar. What is the temperature? 

b) The pressure is changed until the contents become 100% saturated vapor. What is that pressure?

c) With pressure held constant at 45 bar, steam is added to or removed from the tank, as needed, until the contents are 25% liquid by volume. Calculate the mass of steam that must be added or removed and report it as a percentage of the total mass originally in the tank.