Set the multiplier and say which of the two speeds you were given. The solver flips n into the speed ratio, adds and subtracts for the master ratio, anchors on your real number, and then rebuilds D and U to prove the multiplier comes back out.
3 times as long back, stream 4 km/h
b : s = 2 : 1 · b = 8 km/h
Set the multiplier to 1 — equal times both ways. The formula would divide by zero, so the solver explains why no river fits instead of printing a speed.