Speed of a boat in still water is three times the speed of the stream. Time taken by the boat to travel 180 Km downstream is 9 hours. Find the respective ratio of the time taken by the boat to travel 80 Km downstream and 50 Km upstream?
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5:6
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4:5
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3:4
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2:3
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None of these
B
Correct answer
Explanation
Let stream speed = x, then still water speed = 3x. Downstream speed = 4x. Given 180 km in 9 hours downstream: 4x = 180/9 = 20 kmph, so x = 5 kmph. Upstream speed = 2x = 10 kmph. Time for 80 km downstream = 80/20 = 4 hours. Time for 50 km upstream = 50/10 = 5 hours. Ratio = 4:5. Option A (5:6) results from mixing up downstream and upstream times.