C3H8 (g) + 5 O2 (g) => 3 CO2 (g) + 4 H2O (g)
Propane will burn in oxygen to produce carbon dioxide and water. So let's write down the initial equation
C3H8 + O2 => CO2 + H2O
Now count the number of atoms of each type for each side of the equation
C 3 1
H 8 2
O 2 3
Obviously they don't match. So let's look closer. I see 3 carbon atoms on the left side and only CO2 on the right consume the carbon. So let's make the equation show 3 CO2 molecules. And since I have 8 hydrogen atoms on the left and 2 on the right, let's also make the equation make 4 water molecules. So we now get
C3H8 + O2 => 3 CO2 + 4 H2O
Now let's count the number of atoms on both sides
C 3 3
H 8 8
O 2 10
The carbon and hydrogen look good. But we need more oxygen on the left side. Looks like 5 oxygen molecules will work fine. So we get
C3H8 + 5 O2 => 3 CO2 + 4 H2O
Count the atoms again
C 3 3
H 8 8
O 10 10
And everything balances. All of the reactants and products are gasses during the burning so the final balanced equation is
C3H8 (g) + 5 O2 (g) => 3 CO2 (g) + 4 H2O (g)
