Question 1129173
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Since each way "to the park" and "back" is the same distance, 
the travel in one way at 10 mph took 2 times as much time as the travel back at 20 mph.


And the sum of these two times is 6 hours.


So the answer is OBVIOUS:  4 hours ar 10 mph  and  2 hours at 20 mph.
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Solved.