I don't think it's about difficulty per se but the number of computations required. If 25 miles were covered by the fly and a bicycle, the fly covers 15 making it 60% (first calculation). Thus for 20 miles, the fly covers 12, bicycle 8 (second calculation). 20-16 or 4 miles are remaining of which the fly covers 2.4 (third computation). 2.4 is one-fifth of 12 (fifth computation). 12 / 0.8 => 12 * 5/4 = 15 (sixth computation). This route will always be slower because it requires more computations.

Where intuition really comes in is finding the right hill to climb - http://en.wikipedia.org/wiki/Hill_climbing