An average average speed

My average speed for a journey was 50 mph, my return average speed of 70 mph. Why wasn't my average speed for the round trip 60mph ?
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Problem



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An Average Average Speed


I drove from Cambridge to Cardiff at an average speed of 50 mph, it was heavy traffic, but I managed to drive back at an average speed of 70 mph. Why wasn't my average speed for the round trip 60mph ?

What average speed for the return journey would make the round trip average 60 mph ?