The balanced chemical reaction is:
C3H8 + 5O2 = 3CO2 + 4H2O
We are given the amount of C3H8 to be used for the reaction. This will be the starting point for the calculations.
73.7 g C3H8 (1 mol C3H8 / 44.11 g C3H8) ( 5 mol O2 / 1 mol C3H8 ) ( 32 g O2 / 1 mol O2 ) = 267.3 g O2