Balance the equation: C3H8 + O2 -> CO2 + H2O.

Balance carbon first: C3H8 needs 3CO2.
Balance hydrogen next: 8 H atoms need 4H2O.
Now count oxygen on the products: 3CO2 has 6 O atoms and 4H2O has 4 O atoms, for 10 O atoms total.
Use 5O2 on the reactant side.
The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O.