Answer :
It sounds like Joe calculated 5/400 = 0.0125
He divided in the wrong order. It should be 400/5 = 80 since we divide distance over time to get the speed (or rate).
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In terms of x and y, we basically are saying
slope = rise/run
slope = (change in y)/(change in x)
slope = (change in distance)/(change in time)
slope = (400 km)/(5 hrs)
slope = 80 km per hour is the speed