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      Question

      The speed of boat 'X' in upstream is 18 km/hr and speed

      of boat 'Y' in downstream is 30 km/hr. If the difference between speeds of boat 'X' and 'Y' in still water is 20 km/hr, then find the time taken by boat 'X' to cover 210 kilometres in downstream. (Note: speed of boat 'X' in still water is more than that of boat 'Y' and both boats are travelling in same river)
      A 3.5 hrs Correct Answer Incorrect Answer
      B 4.2 hrs Correct Answer Incorrect Answer
      C 5.5 hrs Correct Answer Incorrect Answer
      D 7.2 hrs Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let speed of boat 'X' in still water be 'x' km/hr Therefore, speed of boat 'Y' in still water = (x - 20) km/hr According to the question, (x - y) = 18 ...... (I) And, (x - 20) + y = 30 Or, (x + y) = 50 ...... (II) Adding equation (I) and (II), we get 2x = 68 Or, 'x' = (68 / 2) = 34 Putting value of 'x' in equation (I), we get 'y' = 16 Therefore, downstream speed of boat 'X' = 34 + 16 = 50 km/hr Required time taken = (210 / 50) = 4.2 hours

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