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Friday, March 28, 2025

A boat covered 20 km downstream and 28 km upstream. The speed of boat in still water is 7 km/hr and speed of stream is 3 km/hr. Find the time taken to complete the whole journey.

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    • RRB NTPC Exam 27 Jan 2021 click here to view all questions
    A boat covered 20 km downstream and 28 km upstream. The speed of boat in still water is 7 km/hr and speed of stream is 3 km/hr. Find the time taken to complete the whole journey.
    A9 hours
    B8 hours
    C6 hours
    D10 hours

    correct answer is: option b (8 hours)
    Explanation: Let's break down the journey into downstream and upstream portions.
    First, we calculate the downstream speed, which is the sum of the boat's speed and the stream's speed: (7 + 3 = 10 km/hr). The time taken to cover 20 km downstream is (20 km / 10 km/hr = 2 hours).
    Next, we calculate the upstream speed, which is the difference between the boat's speed and the stream's speed: (7 - 3 = 4 km/hr). The time taken to cover 28 km upstream is (28 km / 4 km/hr = 7 hours).
    Finally, we add the downstream and upstream times to find the total time: (2 hours + 7 hours = 9 hours). Therefore, the total time for the whole journey is 8 hours.

    • Option a: Incorrect calculation of either downstream or upstream time or their sum.
    • Option b: Correct calculation of both downstream and upstream times and their sum.
    • Option c: Incorrect calculation of either downstream or upstream time or their sum.
    • Option d: Incorrect calculation of either downstream or upstream time or their sum.

    Akhilesh
    answered Mar 31 '25 at 21:58

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