Question 132505
135/15=9 mph. for the trip upstream.
135/(15-4)=135/11=12.27 mph. for the trip downstream.
(12.27-9)/2=3.27/2=1.64 mph. for the current.
9+1.64=10.64 mph. for the boat ibn still water.
12.27-1.64=10.64 mph for the boat.