Question 1091568
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d = r * t    where r=rate, t=time, d=distance
so yes, rearranging to t = d/r "time varies inversely with speed if distance is constant"

The trip is the same distance, no matter what the speed.  Can we find the trip distance?  Yes, from the speed and time information given (4 hrs at 80km/hr):  

d = 4hrs * 80km/hr = 320km

Now we can divide the trip distance by the other speed in order to get the time required:

t = 320km / 64km/hr = {{{ highlight(5) }}} hrs
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A 2nd way to do this (although mathematically equivalent) is to use the ratio of the two speeds, essentially it is just a re-grouping the distance calculation, as follows:

 t = (4hrs * 80km/hr) / 64km/hr = <b>4hrs * (80km/hr / 64km/hr)</b>  = 4hrs * (1.25) = 5 hrs


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Hope this helps!