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A boat takes 3hr to travel 30 km down a river, then 5hr to return. How fast is the river flowing?

  • physics -

    Let V = stream velocity and v = boat velocity with respect to water. Both are in km/hr.

    3 hr = 30/(V + v)
    5 = 30/(V -v)

    Solve those two equations in two unknowns for V and v.
    5(V -v) = 3(V +v)
    2V = 8v
    v = V/4

    3 = 30/(5V/4)= 120/5V
    15V = 120


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