Question 1092797
distance= speed* time
Suppose Selfoss is 80 km away
it takes 1 hour to get there and 4/3 of an hour to return.
That would be 7/3 of an hour to drive 160 km
that is 160*3/7 km/h or 68.57 km/h.
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Suppose Selfoss is 60 km away.  
It takes 3/4 hour to get there and 1 hour to get back or 7/4 hour to go 120 km
that is 120*4/7 or again 68.57 km/h average speed.