remember that we have to have the same amount of atoms on each side of the equation. start with C there is 1 on the left and 8 on the right so 8 has to be on the CO. now there is an 8 now on the water there is 1 originally and now we need 8. from water there is now a total of 16 hydrogen plus the 18 from octane for a total of 34. the left has an H2. 34/2 = 17. that is where the 17 comes from
