Question 1065804
A different perspective:
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The boat and the bottle are in the same medium, the river, so they would stay together if not for the man rowing.
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Instead of a bridge, envision this happening in open water near a marker, such as a buoy.  The bottle stays near the marker.
The man rows 20 minutes away, so it takes him 20 minutes to row back, 40 minutes total.
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The bottle in the river moves 1 mile in 40 minutes --> 1.5 mi/hr.
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Choosing a frame of reference and remembering that "all motion is relative" makes it a simple problem.