Set the two leg times and the one speed you were told, and the solver walks the chain: flip, add and subtract, anchor, then check the implied distance both ways. Set the upstream time below the downstream one and it will explain why that cannot happen.
24 h down, 32 h up, stream 6 km/h
D : U = 4 : 3 · b : s = 7 : 1 · b = 42
Try 24 hours down and 4 hours up. Coming back would be the faster direction, which no current can arrange, so the solver refuses the question.