If the delta G for the oxidation of NADH by oxygen is 220 kJ

If the delta G for the oxidation of NADH by oxygen is -220 kJ/mol and the delta G for transport of H⁺ across the inner membrane is 20 kJ/mol, what is the efficiency for energy captured in the proton gradient versus energy evolved from NADH oxidation?

The answer is 90%, could anyone explain it? Thank you

Solution

energy= delta G for oxidation of NADH by oxygen/Delta G for transport of H+

          =220/20=110

EFFICIENCY FOR ENERGY CAPTURED= energy- delta G for H+

                                                           =110-20=90%