Refer to figure.
The GCT approximates to a straight line on a lambert conformal chart. Between two points on the chart, the GCT will change by the convergency.

Lambert chart: Convergency = Change of longitude º x Convergence factor
Where convergence factor equals sin of Parallel of Origin.

Parallel of Origin: half way between standard parallels - in this case, standard parallels 40ºN and 55ºN => Parallel of origin is (40º + 55º)/2 = 47.5ºN
Convergence factor = Sin 47.5º = 0.737

• Convergency = 25º x 0.737 = 18.425º (25 longitude change is from 10°E to 15°W)

We are on the Northern hemisphere and flying to the east, we have to add the convergency to the track we are flying to obtain the track on arrival:

• Arrival track at Aalborg = 068º + 18º = 086º(T)