Joe and Sally tend to argue when they travel. Just as they reached the moving sidewalk at the airport, their struggle for itinerary-making powers peaked. Though they stepped on the moving belt at the same time, Joe chose to stand and ride, while Sally opted to keep walking. Sally reached the end in 1 min, while Joe took 2 min. How long would it have taken Sally if she had walked twice as fast?

Let v_B = velocity of belt, v_S velocity of Sally when walking normally, and D the length of the belt.