Given the limited capacity of his transport vehicle (20 head of cattle), the 400 head of cattle will require 400/20=<<400/20=20>>20 trips using his transport vehicle.
Traveling to the site at 60 mph for 60 miles it will take 60/60=<<60/60=1>>1 hour to travel one-way.
Since each trip requires driving to and returning from the relocation site, each complete round trip will take 2*1=<<2*1=2>>2 hours.
Thus, 20 complete trips will take 20*2=<<20*2=40>>40 hours of driving time.
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