Aptitude - Boats and Streams - Discussion
Discussion Forum : Boats and Streams - General Questions (Q.No. 8)
8.
A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:
Answer: Option
Explanation:
Let the speed of the stream x mph. Then,
Speed downstream = (10 + x) mph,
Speed upstream = (10 - x) mph.
 |
36 | - | 36 | = | 90 |
| (10 - x) | (10 + x) | 60 |
72x x 60 = 90 (100 - x2)
x2 + 48x - 100 = 0
(x+ 50)(x - 2) = 0
x = 2 mph.
Discussion:
85 comments Page 6 of 9.
Deepesh said:
9 years ago
@Chandhu.
Time taken = time taken upstream + time taken downstream.
i.e.
90/60 hrs = (18/10-x)+(18/10+x).
Then, why 90/60 hrs = (36/10-x)-(36/10+x)?
Time taken = time taken upstream + time taken downstream.
i.e.
90/60 hrs = (18/10-x)+(18/10+x).
Then, why 90/60 hrs = (36/10-x)-(36/10+x)?
Vijay said:
8 years ago
36(10 + x) - 36(10 - x) = 90\60.
How to solve this?
How to solve this?
(1)
Oni said:
8 years ago
36/(10-x)-36/(10+x) = 90/60,
=> 36/(10-x)-36/(10+x) = 3/2,
=> 12/(10-x)-12/(10+x) = 1/2,
=> 12(10+x)-12(10-x)/(10-x)(10+x) = 1/2,
=> 24(10+x)-24(10-x) = (10-x)(10+x),
=> 48x = 100-x^2,
=> x^2+50x-2x-100 = 0,
=> (x+50)(x-2) = 0,
=> x = 2.
=> 36/(10-x)-36/(10+x) = 3/2,
=> 12/(10-x)-12/(10+x) = 1/2,
=> 12(10+x)-12(10-x)/(10-x)(10+x) = 1/2,
=> 24(10+x)-24(10-x) = (10-x)(10+x),
=> 48x = 100-x^2,
=> x^2+50x-2x-100 = 0,
=> (x+50)(x-2) = 0,
=> x = 2.
Soumajit Bose said:
8 years ago
If the boat takes 90 minutes more, then what shall we do?
Harshi said:
8 years ago
36/ (10-x) -36/ (10+x).
Why had we done subtraction here? it means why won't we did addition? Please explain.
Why had we done subtraction here? it means why won't we did addition? Please explain.
Diwakar said:
8 years ago
1 mile = 1600 meters = 1.6 km.
Manjunath said:
8 years ago
Why had we done subtraction here? it means why won't we did addition? Please explain.
Sriteja said:
8 years ago
Why have we done addition here instead of subtraction? Please explain.
Karthikeyan Vaithilingam said:
8 years ago
There can be 2 answer if we consider (x+50)(x-2) = 0.
x can be 2 or x can be -50.
I think speed cannot be in negative as only 2 is the valid answer.
x can be 2 or x can be -50.
I think speed cannot be in negative as only 2 is the valid answer.
Revathi said:
7 years ago
Why are we subtracting downstream and upstream? Please explain.
(1)
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