1) First let's rewrite the equation:
O₂ + C₈H₁₈ → CO₂ + H₂O
To balance the equation, we need to equal the number of atoms of each element on the reactants and products side. If you have 8 atoms of carbon on the reactants side, you need to have 8 atom on the products side as well. For this, ou put an eight on the left of the CO₂:
O₂ + C₈H₁₈ → 8 CO₂ + H₂O
You do the same thing for the rest of the elements.
25/2 O₂ + 1 C₈H₁₈ → 8 CO₂ + 9 H₂O
25/2 is a fraction. If you want the answer with just whole numbers, you multiply the whole equation by 2:
25 O₂ + 2 C₈H₁₈ → 16 CO₂ + 18 H₂O
So the coefficient of O₂ is 25.