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A Martin Gardner problem

  1. Nov 22, 2007 #1
    post your answers in white :biggrin: . no pen or paper allowed.

    Two ferryboats start at the same instant from opposite sides of a river, traveling across the water on routes at right angles to the shores. Each travels at a constant speed, but one is faster than the other. They pass at a point 720 yards from the nearest shore. both boats remain in their slips for 20 minutes before starting back. On the return trips they meet 400 yards from the other shore.

    How wide is the river?


    I didn't get the right answer and then I felt like an idiot because the answer is really easy :frown: .
     
  2. jcsd
  3. Nov 23, 2007 #2
    The answer is:
    1760
    yards
     
  4. Nov 23, 2007 #3
    Well,
    at the two moments the ferryboats meet, the relation between the distances covered by them is the same:
    (w - a) / a = (2w - b) / (w + b)
    So,
    w = 3a - b = 3*720 - 400
    w = 1760 yards

    :smile:
     
  5. Nov 27, 2007 #4
    oops, totally forgot about this thread— yep, both right :biggrin: .
     
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