At a constant temperature and number of moles of the gas the product of PV is equal to some constant this is Boyle's law. At another set of condition of the same temperature, the constant is still the same. Calculations are as follows:
P1V1 =P2V2
V2 = P1 x V1 / P2
V2 = 270 x 1 / 540
V2 =0.5 L