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Speed of a boat in still water is 40 kmph and it takes 5 hours for it to traverse a river 300 km wide while going with the stream. What will be the difference in time for the boat between the forward and the return journey?1. 5 hrs2. 10 hrs3. 15 hrs4. 20 hrs5. None of these |
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Answer» Correct Answer - Option 2 : 10 hrs Given: Speed of the boat in still water = 40 kmph Total distance = 300 km Concept: This is a simple question of boat and stream. We need to remember that, Going along the stream, resultant speed = speed of the boat in still water + speed of the stream Going against the stream, resultant speed = speed of the boat in still water - speed of the stream Formula used: s = v × t Where s = distance, v = speed of the boat, t = time taken Calculation: Effective speed of the boat along the stream = 300/5 = 60 kmph So, speed of the stream = 60 - 40 kmph = 20 kmph While returning, Effective speed = 40 - 20 = 20 kmph Time required = 300/20 = 15 hrs Required difference = 15 - 5 = 10 hrs. Students often make a mistake of not properly reading the question and puts 15 hrs as the answer as it is also in the option. |
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