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      Question

      A motorboat moves downstream at 56 km/h. After

      completing half of the route, the stream’s speed rises by 25%. Because of this, the boat reaches 20 minutes earlier than the usual time. If the total distance is 560 km, find the original speed of the stream.
      A 18 km/h Correct Answer Incorrect Answer
      B 10 km/h Correct Answer Incorrect Answer
      C 16 km/h Correct Answer Incorrect Answer
      D 21 km/h Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let the speed of the stream be ‘x’ km/h. Original time for the whole journey = 560/56 = 10 hours So, time taken to cover the first half = 10 ÷ 2 = 5 hours New total time (because of early arrival) = 10 hours − 20 minutes = 9 hours 40 minutes = 9.666… hours So, time taken for the second half now = 9.666… − 5 = 4.666… hours (i.e., 4 hours 40 minutes) So, speed in the second half = 280 / 4.666… = 60 km/h Net change in speed = 60 − 56 = 4 km/h Increase due to stream = 1.25x − x = 0.25x So, 0.25x = 4 ⇒ x = 16 Therefore, the original speed of the stream = 16 km/h

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