Aptitude - Boats and Streams - Discussion
Discussion Forum : Boats and Streams - General Questions (Q.No. 12)
12.
A man can row three-quarters of a kilometre against the stream in 11
minutes and down the stream in 7
minutes. The speed (in km/hr) of the man in still water is:
minutes and down the stream in 7 minutes. The speed (in km/hr) of the man in still water is:Answer: Option
Explanation:
We can write three-quarters of a kilometre as 750 metres,
and 11 minutes as 675 seconds.
| Rate upstream = |  |
750 |  m/sec |
= | 10 | m/sec. |
| 675 | 9 |
| Rate downstream = | |
750 | m/sec |
= | 5 | m/sec. |
| 450 | 3 |
 Rate in still water = |
1 | |
10 | + | 5 | m/sec |
| 2 | 9 | 3 |
| = | 25 | m/sec |
| 18 |
| = | |
25 | x | 18 | km/hr |
| 18 | 5 |
= 5 km/hr.
Discussion:
42 comments Page 3 of 5.
Naveena said:
9 years ago
Hi, I'm a beginner for this type of problem. Can anyone explain this question for me?
Dhakshina said:
10 years ago
Let us consider distance as 3/4 km.
Upstream speed is 45/4.
Speed travelled is d/t. So (3/4)/(45/4)*1/60 (convert this into hours) = 4 km/hr.
Downstream speed is 15/2.
Speed travelled is d/t therefore (3/4)/(15/2)*1/60 = 6 km/hr.
Therefore to find the speed of still water (Upstream + Downstream)/2 = (4+6)/2 = 5 km/hr.
Upstream speed is 45/4.
Speed travelled is d/t. So (3/4)/(45/4)*1/60 (convert this into hours) = 4 km/hr.
Downstream speed is 15/2.
Speed travelled is d/t therefore (3/4)/(15/2)*1/60 = 6 km/hr.
Therefore to find the speed of still water (Upstream + Downstream)/2 = (4+6)/2 = 5 km/hr.
(2)
Sudarshan said:
10 years ago
Man Speed = 1/2 (Upstream Speed + Downstream Speed).
= 1/2 (750*4/45*60 + 750*2/15*60).
= (1/2)*(750/60)(2/15)( 2/3 + 1).
= (750*2/2*60*15) * (5/3).
= 25/18 meters/sec.
= (25/18)*(18/5) Km/hr.
= 5 Km/hr.
= 1/2 (750*4/45*60 + 750*2/15*60).
= (1/2)*(750/60)(2/15)( 2/3 + 1).
= (750*2/2*60*15) * (5/3).
= 25/18 meters/sec.
= (25/18)*(18/5) Km/hr.
= 5 Km/hr.
Buny said:
10 years ago
How to find time of download stream? How it is calculated 15/2?
Nikul Rana said:
10 years ago
Let we assume the speed of man in still water = X kmph;
And the speed of water current = Y kmph;
So, The speed downstream = X+Y;
And the speed against the stream = X-Y;
Therefor we can say that X-Y = (3/4)/(45/4) = 4;
X-Y = 4.....(1);
And X+Y = (3/4)/(15/2) = 6;
X+Y = 6.....(2);
So that we get X = 5; //The speed of the man in still water.
And the speed of water current = Y kmph;
So, The speed downstream = X+Y;
And the speed against the stream = X-Y;
Therefor we can say that X-Y = (3/4)/(45/4) = 4;
X-Y = 4.....(1);
And X+Y = (3/4)/(15/2) = 6;
X+Y = 6.....(2);
So that we get X = 5; //The speed of the man in still water.
Bijendra Singh said:
1 decade ago
Downstream time is given there 7 1/2 so it is 15/2*60.
i.e. 900/2 = 450
i.e. 900/2 = 450
Sunil said:
1 decade ago
How did you find rate of downstream ?
Neelam said:
1 decade ago
How we find downstream time as it is not mention in the question?
Fahmi said:
1 decade ago
We can do it more easy way.
(3/4) * (4/45) *60 = 4 km.
(3/4) * (2/15) *60 = 6 km.
1/2 (4+6) km = 5 km.
(3/4) * (4/45) *60 = 4 km.
(3/4) * (2/15) *60 = 6 km.
1/2 (4+6) km = 5 km.
Kishor said:
1 decade ago
Downstream : Upstream
...................................................
Distance(D) 3/4 x 1 km : 3/4 x 1 km
Time(T) 15/2 x 60 hr : 45/4 x60 hr
Speed(D/T) (3/4 x1)/(15/2 x60)=6 : (3/4 x1)/(45/4 x60)=4
Speed of man in still water = (6+4)/2 = 5 km/hr.
...................................................
Distance(D) 3/4 x 1 km : 3/4 x 1 km
Time(T) 15/2 x 60 hr : 45/4 x60 hr
Speed(D/T) (3/4 x1)/(15/2 x60)=6 : (3/4 x1)/(45/4 x60)=4
Speed of man in still water = (6+4)/2 = 5 km/hr.
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