Balance: C3H8 + O2 -> CO2 + H2O

Step 1 — Balance the carbon

Propane carries three carbon atoms, and every one of them ends up in carbon dioxide:

C₃H₈ + O₂ → 3 CO₂ + H₂O

Carbon is done and will not be touched again.

Step 2 — Balance the hydrogen

Eight hydrogen atoms on the left, two per water molecule on the right:

8 ÷ 2 = 4 → C₃H₈ + O₂ → 3 CO₂ + 4 H₂O

Step 3 — Balance the oxygen last

Oxygen is left until the end because it appears in both products:

That is 10 oxygen atoms, supplied by 5 O₂ molecules:

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

Check

Both sides match, and no coefficient shares a common factor, so the equation is balanced in its lowest whole numbers.