Determine the rate of communication line
The dog could carry 21 gigabytes or 168 gigabits. A speed of 18 km/hour that is equal to 0.005 km/sec. The time to travel the distance x km is x/0.005 = 200x sec, yielding data rate of 168/200x Gbps or 840/x Mbps. For x< 5.6 km, dog has a higher rate than communication line.