The speed of a stream is 3 mph. A boat travels 4 miles upstream in the same time it takes it to travel 10 miles downstream. What is the speed of the boat in still water? Solution D=RT T=D/R T=4/(R-3) T=10/(R+3) 4/(R-3)=10/(R+3) 10(R-3)=4(R+3) 10R-30=4R+12 10R-4R=12+30 6R=42 R=42/6 R=7 MPH IS THE SPEED OF THE BOAT. PROOF: 4/(7-4)=10/(7+3) 4/4=10/10 1=1.