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      Question

      A boat takes a total of 6 hours to travel 45 km downstream

      and 30 km upstream. In another scenario, the same boat takes 8 hours to cover 60 km upstream and 30 km downstream. Determine the speed of the stream (in km/h).
      A 2.5 km/h Correct Answer Incorrect Answer
      B 3.5 km/h Correct Answer Incorrect Answer
      C 1.5 km/h Correct Answer Incorrect Answer
      D 4.5 km/h Correct Answer Incorrect Answer

      Solution

      Let the upstream and downstream speed of boat be 'U' km/h and 'D' km/h respectively.
      So, (45/D) + (30/U) = 6 --------- (I)
      And, (30/D) + (60/U) = 8 -------- (II)
      On solving 2 X equation I - equation II,
      We get, 2 X [(45/D) + (30/U) ] - [(30/D) + (60/U) ] = 2 X 6 - 8
      Or, (90/D) + (60/U) - (30/D) - (60/U) = 12 - 8
      Or, (60/D) = 4
      Or, 'D' = 15
      On putting value of 'D' in equation I,
      We get, (45/15) + (30/U) = 6
      Or, 3 + (30/U) = 6
      Or, (30/U) = 3
      Or, 'U' = 10
      Speed of stream = (1/2) X (downstream speed - upstream speed)
      = (1/2) x (15 - 10) = 2.5 km/h

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