Set the two journeys. The solver checks that they are independent, writes both in 1/U and 1/D, scales them to a common upstream distance, subtracts to get D, back-substitutes for U, and converts to the boat and the stream only at the end.
8 up + 12 down in 3 h, 12 up + 24 down in 5 h
D 12 · U 4 · b 8, s 4
Set trip 1 to 30 up, 45 down, 9 hours and trip 2 to 40 up, 60 down, 12 hours. The second is the first multiplied by 4/3, so it carries no new information — and the solver says so instead of inventing an answer.