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 2 of 7.
Sunil sharma, kaverinagar said:
1 decade ago
How can you multiply in place of addition ?
Varun said:
1 decade ago
Here only one direction distance is given. Time which requires to go and back position is given. How is possible to put total time to only one direction distance.
Jagu said:
1 decade ago
How we get 9/2 there please tell me.
Neha said:
1 decade ago
By solving the fraction 4*2=8, n 8+1=9, so the ans is 9/2
Sourabh said:
1 decade ago
Very well Explained Seetha ...Thank you.....
Abhishek said:
1 decade ago
Thank You...!
Seetha
Seetha
Arun.D said:
1 decade ago
Speed in still water = 15 km/hr.
Distance traveled in downstream = 30 km.
Time taken in upstream = 4 hr 30 min.
The speed of the stream = ?
Speed = Distance/time.
Distance = 30km.
Speed in upstream = (u-v)km/hr = (15-v)km/hr.
Speed in downstream = (u+v)km/hr = (15+v)km/hr.
Total time = (distance of downstream/speed of downstream) + (distance of upstream/speed of upstream).
(30/(15+v))+(30/(15-v)) = 4 1/2.
(30*30/(15-v)(15+v)) = 4 1/2.
900/(15^2 - v^2) = 4 1/2.
900/(225 - v^2) = 4 1/2.
900 / (225 - v^2) = 9/2.
900 * 2 = 9(225 -v^2).
1800 = 2025 - 9v^2.
9v^2 = 2025 - 1800.
9v^2 = 225.
v^2 = 225/9.
v^2 = 25.
v = 5 Km/hr.
Distance traveled in downstream = 30 km.
Time taken in upstream = 4 hr 30 min.
The speed of the stream = ?
Speed = Distance/time.
Distance = 30km.
Speed in upstream = (u-v)km/hr = (15-v)km/hr.
Speed in downstream = (u+v)km/hr = (15+v)km/hr.
Total time = (distance of downstream/speed of downstream) + (distance of upstream/speed of upstream).
(30/(15+v))+(30/(15-v)) = 4 1/2.
(30*30/(15-v)(15+v)) = 4 1/2.
900/(15^2 - v^2) = 4 1/2.
900/(225 - v^2) = 4 1/2.
900 / (225 - v^2) = 9/2.
900 * 2 = 9(225 -v^2).
1800 = 2025 - 9v^2.
9v^2 = 2025 - 1800.
9v^2 = 225.
v^2 = 225/9.
v^2 = 25.
v = 5 Km/hr.
Mohanraj said:
1 decade ago
Friends this step is little confused any can help please?
(30/(15+v))+(30/(15-v)) = 4 1/2.
(30*30/(15-v)(15+v)) = 4 1/2.
(30/(15+v))+(30/(15-v)) = 4 1/2.
(30*30/(15-v)(15+v)) = 4 1/2.
Asha said:
1 decade ago
How can calculate distance of upstream ?
Vinay said:
1 decade ago
How can calculate distance of upstream ?
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