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    Question

    The speed of the boat in still water is 20% less than

    the speed of the boat in downstream. The time taken by the boat to cover 780 km distance in upstream is (t+3) hours. If the speed of the stream is 20 km/h, then find out the value of тАШtтАЩ.
    A 10 Correct Answer Incorrect Answer
    B 15 Correct Answer Incorrect Answer
    C 60 Correct Answer Incorrect Answer
    D 20 Correct Answer Incorrect Answer
    E none of these Correct Answer Incorrect Answer

    Solution

    ATQ, Let тАШBтАЩ be the speed of the boat in still water and тАШSтАЩ the speed of the stream, with S = 20 km/h. B = (100-20)% of (B+S) = 80% of (B+S) = 0.8(B+S). B = 0.8(B+20), which leads to B = 0.8B + 16, thus 0.2B = 16, giving us B = 80 km/h. With the boat covering 780 km upstream, where upstream speed = B - S = 80 - 20 = 60 km/h, we have 780/(t+3) = 60. Solving for t yields t+3 = 780/60 = 13, Hence, t = 10. Value of тАШtтАЩ = 10.

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