Boats and Streams Questions Answers

• 8. If a boat goes 7 km upstream in 42 minutes and the speed of the stream is 3 kmph, then the speed of the boat in still water is

  1. 12 kmph
  2. 13 kmph
  3. 14 kmph
  4. 15 kmph

  Answer And Explanation

  Answer: Option B

  Explanation:

  Rate upstream = (7/42)*60 kmh = 10 kmph.
  Speed of stream = 3 kmph.
  Let speed in sttil water is x km/hr
  Then, speed upstream = (x —3) km/hr.
  x-3 = 10 or x = 13 kmph

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• 9. A man takes twice as long to row a distance against the stream as to row the same distance in favour of the stream. The ratio of the speed of the boat in still water and stream is

  1. 3:1
  2. 1:3
  3. 2:4
  4. 4:2

  Answer And Explanation

  Answer: Option A

  Explanation:

  Let speed downstream = x kmph
  Then Speed upstream = 2x kmph
  
  So ratio will be,
  (2x+x)/2 : (2x-x)/2
  => 3x/2 : x/2 => 3:1

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• 10. A man's speed with the current is 20 kmph and speed of the current is 3 kmph. The Man's speed against the current will be

  1. 11 kmph
  2. 12 kmph
  3. 14 kmph
  4. 17 kmph

  Answer And Explanation

  Answer: Option C

  Explanation:

  If you solved this question yourself, then trust me you have a all very clear with the basics of this chapter.
  
  If not then lets solve this together.
  
  Speed with current is 20,
  speed of the man + It is speed of the current
  Speed in still water = 20 - 3 = 17
  
  Now speed against the current will be
  speed of the man - speed of the current
  = 17 - 3 = 14 kmph

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• 11. A boat can travel with a speed of 16 km/hr in still water. If the rate of stream is 5 km/hr, then find the time taken by the boat to cover distance of 84 km downstream.

  1. 4 hours
  2. 5 hours
  3. 6 hours
  4. 7 hours

  Answer And Explanation

  Answer: Option A

  Explanation:

  It is very important to check, if the boat speed given is in still water or with water or against water. Because if we neglect it we will not reach on right answer. I just mentioned here because mostly mistakes in this chapter are of this kind only.
  
  Lets see the question now.
  Speed downstream = (16 + 5) = 21 kmph
  
  Time = distance/speed = 84/21 = 4 hours

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• 12. A man can row at 5 kmph in still water. If the velocity of the current is 1 kmph and it takes him 1 hour to row to a place and come back. how far is that place.

  1. .4 km
  2. 1.4 km
  3. 2.4 km
  4. 3.4 km

  Answer And Explanation

  Answer: Option C

  Explanation:

  Let the distance is x km
  Rate downstream = 5 + 1 = 6 kmph
  Rate upstream = 5 - 1 = 4 kmph
  then
  x/6 + x/4 = 1 [because distance/speed = time]
  => 2x + 3x = 12
  => x = 12/5 = 2.4 km

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• 13. The speed of a boat in still water is 15 km/hr and the rate of current is 3 km/hr. The distance travelled downstream in 12 minutes is

  1. 1.6 km
  2. 2 km
  3. 3.6 km
  4. 4 km

  Answer And Explanation

  Answer: Option C

  Explanation:

  Speed downstreams =(15 + 3)kmph
  = 18 kmph.
  Distance travelled = (18 x 12/60)km
  = 3.6km

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• 14. Sahil can row 3 km against the stream in 20 minutes and he can return in 18 minutes. What is rate of current ?

  1. 1/2 km/hr
  2. 1/3 km/hr
  3. 2 km/hr
  4. 4 km/hr

  Answer And Explanation

  Answer: Option A

  Explanation:

  \begin{aligned}
  \text{Speed Upstream} = \frac{3}{\frac{20}{60}} = 9 km/hr \\
  \text{Speed Downstream} = \frac{3}{\frac{18}{60}} = 10 km/hr \\
  \text{Rate of current will be} \\
  \frac{10-9}{2} = \frac{1}{2} km/hr
  \end{aligned}

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