Set the boat, the current and the downstream time. The solver converts to travelling speeds, multiplies out the shared distance, divides it back for the return leg, and then shows the one-multiplication route that never needs the distance at all.
b 20, s 12, 16 hours downstream
D 32 · U 8 · 512 km · 64 h back
Try a 12 km/h boat on a 15 km/h river. The upstream speed comes out negative, so there is no return journey and the solver says so instead of printing a number.