Combustion is a form of a chemical reaction in which the reactants are oxygen and (usually) a hydrocarbon. The reactants of a combustion reaction are usually water and carbon dioxide.
The first place to start would be to translate the word equation into a chemical equation.
Thus, the combustion of pentene in oxygen to produce water and carbon dioxide becomes:
C5H10 + O2 ---> H2O + CO2
The goal of balancing a chemical equation is to have the same number of atoms of each element on the reactant side as there are on the product side. Currently, there are 5 carbon atoms, 10 hydrogen atoms, and 2 oxygen atoms on the reactant side. On the product side, there are 2 hydrogen atoms, 3 oxygen atoms and 1 carbon atom.
Coefficients are used to add more of the missing atoms. Thus, the final balanced equation would be:
2C5H10 (g) + 15O2 (g) ---> 10CO2 (g) + 10H2O (g)