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Average Speed

Unreviewed import — a classic from the public domain; its solution has not been machine-checked yet.

math
In a recent motor ride it was found that we had gone at the rate of ten miles an hour, but we did the return journey over the same route, owing to the roads being more clear of traffic, at fifteen miles an hour. What was our average speed? Do not be too hasty in your answer to this simple little question, or it is pretty certain that you will be wrong.

Solution

Commit an attempt to unseal the proof. Write your answer — right or wrong, thinking it through is the point.

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