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    Question

    A motorboat moves downstream at 30 km/h over a distance

    of 480 km. After completing half the distance, the stream’s speed becomes 50% higher than before. Because of this, the boat reaches 30 minutes earlier than the usual time. Find the original speed of the stream.
    A 2 km/h Correct Answer Incorrect Answer
    B 4 km/h Correct Answer Incorrect Answer
    C 8 km/h Correct Answer Incorrect Answer
    D 11 km/h Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    ATQ, Let the original speed of the stream be x km/h. Original time = 480/30 = 16 hours Time for first half = 16/2 = 8 hours New total time = 16 − 0.5 = 15.5 hours Time for second half = 15.5 − 8 = 7.5 hours Speed in second half = 240/7.5 = 32 km/h Net increase = 32 − 30 = 2 km/h So, 1.5x − x = 2 0.5x = 2 x = 4

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