Refer to JAR-OPS 1.255
1.3 Contingency fuel, which should be the higher of (a) or (b) below:
a) Either:
i. 5% of the planned trip fuel or, in the event of in-flight replanning, 5% of the trip fuel for the remainder of the flight; or
ii. Not less than 3% of the planned trip fuel or, in the event of in-flight replanning, 3% of the trip fuel for the remainder of the flight, provided that an en-route alternate is available, or
iii. An amount of fuel sufficient for 20 minutes flying time based upon the planned trip fuel consumption provided that the operator has established a fuel consumption monitoring programme for individual aeroplanes and uses valid data determined by means of such a programme for fuel calculation; or
iv. An amount of fuel based on a statistical method approved by the Authority which ensures an appropriate statistical coverage of the deviation from the planned to the actual trip fuel. This method is used to monitor the fuel consumption on each city pair/aeroplane combination and the operator uses this data for a statistical analysis to calculate contingency fuel for that city pair/aeroplane combination.
b) An amount to fly for 5 minutes at holding speed at 1 500 ft (450 m), [6 000 ft (1 800m) for Concorde operations,] above the destination aerodrome in Standard Conditions.
For the purposes of the exam (i.e. answering the original question). It is necessary to use a contingency of 5% (unless told otherwise within the question).