joshii,
I make the answer 1121, as follows:
If the Helicopter travels 90 miles at 150 mph that flight will take 36 minutes. (90/150 =0.6hrs)
The helicopter is in the rig for 15 minutes.
20% faster on the return leg is 180mph, (150 x 1.2). 90/180 = 0.5hrs
Therefore total time in 36+15+30 = 81 minutes = 1hr 21min
This calculation simply involves s=d/t formula and is fairly 'do-able' in your head but as far as using paper in the OASC selection, I've no idea I'm afraid!
Hope this helps & good luck.