1) 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
- 1) 3:1
- 2) 1:3
- 3) 2:4
- 4) 4:2
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Ans. 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
2) 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
- 1) 11 kmph
- 2) 12 kmph
- 3) 14 kmph
- 4) 17 kmph
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Ans. 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
3) 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
- 1) 12 kmph
- 2) 13 kmph
- 3) 14 kmph
- 4) 15 kmph
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Ans. 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 â€”3) km/hr.
x-3 = 10 or x = 13 kmph
4) 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 downstre
- 1) 4 hours
- 2) 5 hours
- 3) 6 hours
- 4) 7 hours
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Ans. 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
5) 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 pla
- 1) .4 km
- 2) 1.4 km
- 3) 2.4 km
- 4) 3.4 km
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Ans. 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
6) A man can row upstream 10 kmph and downstream 20 kmph. Find the man rate in still water and rate of the strea
- 1) 0,5
- 2) 5,5
- 3) 15,5
- 4) 10,5
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Ans. C
Explanation :
Please remember,
If a is rate downstream and b is rate upstream
Rate in still water = 1/2(a+b)
Rate of current = 1/2(a-b)
=> Rate in still water = 1/2(20+10) = 15 kmph
=> Rate of current = 1/2(20-10) = 5 kmph
7) 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
- 1) 1.6 km
- 2) 2 km
- 3) 3.6 km
- 4) 4 km
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Ans. C
Explanation :
Speed downstreams =(15 + 3)kmph
= 18 kmph.
Distance travelled = (18 x 12/60)km
= 3.6km
8) A man takes 3 hours 45 minutes to row a boat 15 km downstream of a river and 2 hours 30 minutes to cover a distance of 5 km upstream. Find the speed of the curren
- 1) 1 km/hr
- 2) 2 km/hr
- 3) 3 km/hr
- 4) 4 km/hr
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Ans. A
Explanation :
First of all, we know that
speed of current = 1/2(speed downstream - speed upstream) [important]
So we need to calculate speed downstream and speed upstream first.
Speed = Distance / Time [important]
\begin{aligned}
\text {Speed upstream =}\\ (\frac{15}{3\frac{3}{4}}) km/hr \ = 15 \times \frac{4}{15} = 4 km/hr \
\text{Speed Downstream = }
(\frac{5}{2\frac{1}{2}}) km/hr \ = 5 \times \frac{2}{5} = 2 km/hr \\text {So speed of current = } \frac{1}{2}(4-2) \ = 1 km/hr
\end{aligned}
9) In one hour, a boat goes 11km along the stream and 5 km against it. Find the speed of the boat in still wat
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Ans. C
Explanation :
We know we can calculate it by 1/2(a+b)
=> 1/2(11+5) = 1/2(16) = 8 km/hr
10) If Rahul rows 15 km upstream in 3 hours and 21 km downstream in 3 hours, then the speed of the stream
- 1) 5 km/hr
- 2) 4 km/hr
- 3) 2 km/hr
- 4) 1 km/hr
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Ans. D
Explanation :
Rate upstream = (15/3) kmph
Rate downstream (21/3) kmph = 7 kmph.
Speed of stream (1/2)(7 - 5)kmph = 1 kmph