A Man And A Dog Problem

A man is taking his dog for a walk. They set off together for a waterfall 10 miles away. They set off exactly together and the man walks at a steady 4 miles an hour. The dog runs off at 8 miles an hour and when it reaches the waterfall it turns round and runs back to the man. When it reaches the man it turns round and runs back to the waterfall and so on until the man and dog arrive at the waterfall.

When did they both arrive at waterfall ?

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