Question 536948
You've got it.  72 miles in 4 hours is 18 mph going downstream.  The same 72 miles upstream takes 6 hours, that's 12 mph.  If the boat's speed is constant and the current is constant, then you can assume the boat goes a constant 15 mph in calm water and is thus sped up 3 mph downstream and slowed down 3 mph upstream.
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Great job,
Lee