The first step in any chemistry problem is making sure that you have a balanced equation. Start with C5H12. If we can, only use 1 of these and try to balance with the rest.
1 C5H12(g)+__O2(g) --------> __CO2(g)+__H2O(g)
If you have 5 C's on the left, then we need 5 on the right.
1 C5H12(g)+__O2(g) --------> 5 CO2(g)+__H2O(g)
Now, balance the H's. 12 on the left, needs to be balanced with 6 H2O on the right
1 C5H12(g)+__O2(g) --------> 5 CO2(g)+ 6 H2O(g)
The last element to balance will be the oxygen. We have 5*2 + 6*1 = 16 on the right, so we need 8 O2
1 C5H12(g)+ 8 O2(g) --------> 5 CO2(g)+ 6 H2O(g)
Now that we have a balanced equation, solving the problem is pretty simple. Since the given value is moles of H2O and we are trying to determing the moles of O2 we will need to use the molar ratio for these two compounds.