Can someone please explain this example

Example 5-1 Estimate the molar volume of an equimolar mixture of methane (1) and propane (2) at T=310.92K,P=206.84 bar and mixtures of 22.1 and 75.3mol percent methane at T=153.15K,P=34.37 bar using CSP. Literature values are 79.34,48.06 and 60.35cm3mol1 respectively (Huang, et al., 1967). PRESSURE-VOLUME-TEMPERATURE RELATIONSHIPS OF MIXTURES 5.7 solution The characteristic properties of methane and propane from Appendix A are listed in the table below. Also, the computed pseudoproperties from Eqs. (5-3.1) to (5-3.3) for the three cases are given. Mole fraction average as in Eq. (5-3.1) + Eq. (5-3.2) \#Eq. (5-3.3) The value of Z can be found from Fig. 4-1 only for the first case, but Tables 3-2 and 3-3 of the 4th Edition give values of Z(0) and Z(1) for the Pitzer-Curl method to use in Eq. (4-3.1), Z=Z(0)+Z(1). The errors for these compressed fluid mixtures with components having significantly different Tc 's and Vc 's are typical for CSP. In general, accuracy for normal fluid mixtures is slightly less than for pure components unless one or more binary interaction parameters are used