Model regresi simultan untuk penjualan LPG PSO dan 12 Kg non PSO studi kasus : Pertamina LPG region IV Jawa Tengah = Simultaneous regression models for sales of LPG PSO and 12 Kg non-PSO case study : region IV Pertamina LPG Central Java - Manuhutu, Romi Ryan, author (2011)