Choose the rower’s speed, the current and a one-way distance. The first three steps walk the four formulas; the last three time both legs and compare the true round-trip average against the still-water speed. Set the stream to zero to see the one case where they agree.
b 12, s 4, 48 km each way
D 16 · U 8 · 9 h · avg 10 2/3
Try a 10 km/h boat on a 12 km/h river. The upstream speed comes out negative, so the solver refuses the question rather than printing an impossible journey.