If you go about this way it is easier to understand. Fuel reqd.13.5+61(planned zfw)=74.5 which is less than 79.0T Max structural (actually WAT limited weight). So in this case the take off weight is restricted by Max Ldg wt. You work backwards MLW limit is 66.4+10(BO)= 76.4. Your take off weight cannot exceed that. So 76.4-61(planned ZFW)=15.4T. This is the maximum fuel you can uplift. So you can take extra 15.4-13.5= 1.9T. Max ZFW is limit by itself don't confuse it with RTOW.