Refer to figure.
ATC endurance is the maximum time that the aircraft can remain airborne taking into account the Final Reserve Fuel consumption as well, i.e. ATC edurance is up to dry tanks or ATC endurance is the elapsed time from take-off at which point you run out of all usable fuel, excluding the start-up and taxi fuel in your calculations.

The question intentionally asks from the students to fill in the table, because the answer comes from the sum of the flight times which correspond to each fuel amount related with the airborne time (excluding taxi fuel).

• The Contingency fuel is: 5% x Trip fuel = 5% x 5 800 kg = 290 kg, which corresponds to a flight time of: 5% x 02:32 = 7.6 min.
• The Final Reserve fuel for a twin jet aeroplane is always fuel to fly for 00:30.
• The Extra fuel is: Ramp fuel - Taxi fuel - Trip fuel - Contingency fuel - Final Reserve fuel - Alternate fuel = 585 kg.
• The fuel flow for the Extra fuel is given as 2 400 kg/hr. So, the Extra fuel of 585 kg will permit to fly for: 585 kg / 2 400 kg/hr = 14.6 min.

Thus, the ATC endurance is the sum of the following flight times: 02:32 + 00:07.6 + 00:42 + 00:30 + 00:14.6 = 04:06.2, which is very close to 04:07.