Aptitude - Boats and Streams - Discussion
Discussion Forum : Boats and Streams - General Questions (Q.No. 4)
4.
A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:
Answer: Option
Explanation:
Let the speed of the stream be x km/hr. Then,
Speed downstream = (15 + x) km/hr,
Speed upstream = (15 - x) km/hr.
 |
30 | + | 30 | = 4 | 1 |
| (15 + x) | (15 - x) | 2 |
 |
900 | = | 9 |
| 225 - x2 | 2 |
9x2 = 225
x2 = 25
x = 5 km/hr.
Video Explanation: https://youtu.be/lMFnNB3YQOo
Discussion:
63 comments Page 5 of 7.
Prabhat said:
9 years ago
Whoever is having confusion about how it got multiplied in place of addition in
(30/(15+x)) + (30/(15-x)) = 9/2 hrs.
It is by cross multiplication it will become,
(30 * (15-x))+(30 * (15+x)) / (15+x) * (15-x) = 9/2 hrs.
After simplifying it will become 900/15^2 - x^2 = 9/2.
(30/(15+x)) + (30/(15-x)) = 9/2 hrs.
It is by cross multiplication it will become,
(30 * (15-x))+(30 * (15+x)) / (15+x) * (15-x) = 9/2 hrs.
After simplifying it will become 900/15^2 - x^2 = 9/2.
P Dixit said:
9 years ago
Put in Formula.
If total Distance = D= 30, Total Time = T= 9/2 Hr., Boat Speed= B = 30, Stream Speed = S=?,
Then,
Distance between the two places is D = T(B^2 - S^2)/2B
S = 5 Ans.
If total Distance = D= 30, Total Time = T= 9/2 Hr., Boat Speed= B = 30, Stream Speed = S=?,
Then,
Distance between the two places is D = T(B^2 - S^2)/2B
S = 5 Ans.
Darshana Jain said:
9 years ago
Thank you very much for the given explanation.
Srinivas said:
8 years ago
Nice explanation @Seetha.
Krishna said:
8 years ago
The answer is 4 * 1/2 = 2.
How is it possible the answer is 9/2. If 4+1/2 then only answer is 9/2.
How is it possible the answer is 9/2. If 4+1/2 then only answer is 9/2.
Rohit said:
8 years ago
How came 4(1/2)?
Praveenbehera said:
8 years ago
Good explanation @Seetha. Thanks.
Priya said:
8 years ago
The speed of the current is 5 then the speed of the boat in downstream =u+v =15+5 =20;.
The speed of the boat in upstream = u-v = 15-5 =10;.
Time =distance/speed.
4 1/2 = 30/20+30/10.
9/2=9/2.
The speed of the boat in upstream = u-v = 15-5 =10;.
Time =distance/speed.
4 1/2 = 30/20+30/10.
9/2=9/2.
Pawan thakur said:
8 years ago
Sneha just like that 4 hours 30 minutes.
Convert 30 minutes 30/60 you will get 1/2.
Now 4 hours 30 minutes = 4*1/2 you will get 9/2 in place of 30 minutes put 1/2.
Convert 30 minutes 30/60 you will get 1/2.
Now 4 hours 30 minutes = 4*1/2 you will get 9/2 in place of 30 minutes put 1/2.
Sowmya.G said:
8 years ago
Speed= 15km/hr
Distance= 30 km/hr
Time= distance/speed
Speed of stream (v) is taken as x
Speed of boat in still water(u)=15km/hr
So,
Speed upstream=(15-x) km/hr.
Speed downstream=(15+x)km/hr.
30/15+x + 30/15-x=4 1/2(this is equation is written with the help of, time= distance/speed)
Further calculations are made. So we get x=5 km/hr.
Distance= 30 km/hr
Time= distance/speed
Speed of stream (v) is taken as x
Speed of boat in still water(u)=15km/hr
So,
Speed upstream=(15-x) km/hr.
Speed downstream=(15+x)km/hr.
30/15+x + 30/15-x=4 1/2(this is equation is written with the help of, time= distance/speed)
Further calculations are made. So we get x=5 km/hr.
(1)
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