Find the BALANCED equation

Gaseous butane (C4H10) combusts with excess oxygen gas to form gaseous water and carbon dioxide gas.

C4H10(g) + O2(g) ---> CO2(g) + H2O(g)

carbon:

C4H10(g) + O2(g) ---> 4CO2(g) + H2O(g)

hydrogen:

C4H10(g) + O2(g) ---> 4CO2(g) + 5H2O(g)

oxygen:

C4H10(g) + (13/2)O2(g) ---> 4CO2(g) + 5H2O(g)

get rid of fraction:

2C4H10(g) + 13O2(g) ---> 8CO2(g) + 10H2O(g)

Done!