Take-off fuel is block fuel (260 kg) minus taxi fuel (10 kg), i.e. 250 kg.
Reserve fuel must not be less than 25 % of the trip fuel, so reserve fuel is (250 kg / 125) · 25 = 50 kg.
Trip fuel is take-off fuel minus reserve fuel, i.e. 250 kg - 50 kg = 200 kg.
After 2 hours of a 3.5 hours’ flight, 2/3.5 of the trip fuel are burnt: 2/3.5 · 200 kg ≈ 114 kg. The remaining trip fuel is hence 200 kg - 114 kg = 86 kg. The 50 kg reserve fuel must not be forgotten, so remaining fuel in tanks is 86 kg + 50 kg = 136 kg.
Last edited by hvogt; 18th Aug 2013 at 08:31.
Reason: grammar