Question 173670
D=RT
R=D/T
R=20/3=6.667 MPH. DOWN STREAM
R=20/4=5 MPH. UP STREAM 
(6.667-5)/2=1.667/2=.833 MPH. FOR THE CURRENT.
6.667-.833=5.833 SPEED OF THE BOAT.
5+.833=5.833 DITTO.