Similar Ideal Gas Law question

[MastersBound]

1. The problem statement, all variables and given/known data
A 500 m^3 tank is filled with saturated liquid-vapor mixture of water at 200 kPa. If 20% percent of mass is liquid and 80 percent of the mass is vapor, the total mass in the tank is?


2. Relevant equations

pV=mRT

3. The attempt at a solution

Don't have a clue... sorry

[vela]

Start by looking up the specific volume of water for the two phases in a table.