Margaret has just enough gas in her speedboat to get to the marina, an upstream journey that takes 4.0 h. Finding it closed for the season, she spends the next 8.0 h floating back downstream to her shack. The entire trip took 12.0 h; how long would it have taken if she had bought gas at the marina?

Let D = distance to marina, v_W = velocity of stream, v_rel = velocity of boat under power relative to stream.