A mixture comprised of 30-mol-% methane, 10-mol-% ethane, 30-mol-% propane, and 30-mol-% n-butane is brought to a condition of 258.15 K (- 15°C) at pressure P, where it exists as a vaporlliquid mixture in equilibrium. If the mole fraction of the methane in the vapor phase is 0.80, what is pressure P (in bar)?