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 they both arrive at waterfall ?