Question 1065804
<pre><b>
Suppose the man in the boat in the beginning is traveling south, 
and when he turns around and comes back he travels north. 

What I'm about to tell you to imagine is really weird, but you 
can imagine it if you try.

Pretend that the river is still and the bank and bridge are
moving.  [Yes, I said it was weird!}  The man at first moves 
south and the bridge moves north.  

Then the bottle stays put and the bridge moves 1 mile north from 
the bottle for 40 minutes, while the man goes south for 20 minutes
and back north to the bottle for another 20 minutes. 

[Remember the bottle has not moved at all in the way we are imagining].

So the bridge has moved 1 mile from the bottle in 40 minutes, That's 
the same as moving 3 miles in 120 minutes or 1.5 miles in 60 minutes 
or 1.5 miles per hour.

The imagined speed of the bridge is the same as the speed of the
current.

Answer 1.5 mph.

Edwin</pre></b>