Balance the equation: C4H8 + O2 goes to CO2 + H2O

Balance the carbon atoms first by adding a 4 in front of the CO2,

Then balance the hydrogen atoms by adding a 4 in front of the H2O, 

Count up the number of oxgens on the right hand side: (42)+(41)=12

Therefore add a 6 before the O2 on the left hand side of the equation