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 3 of 9.
Diwakar said:
8 years ago
1 mile = 1600 meters = 1.6 km.
Kasa said:
9 years ago
Please give me any shortcuts to find the solution.
Needa said:
4 years ago
72x / 100 - x^2 = 90/60.
72x * 60 = 90 *(100-x^2),
72x * 60 = 9000 - 90x^2,
Dividing throughout by 90.
(72x * 60) / 90 = 9000/90 - 90x^2.
48x = 100 - x^2.
x^2 + 48x - 100 = 0.
x^2 +50x - 2x -100 = 0.
x(x + 50) -2(x + 50) = 0.
(x + 50) (x-2) = 0.
72x * 60 = 90 *(100-x^2),
72x * 60 = 9000 - 90x^2,
Dividing throughout by 90.
(72x * 60) / 90 = 9000/90 - 90x^2.
48x = 100 - x^2.
x^2 + 48x - 100 = 0.
x^2 +50x - 2x -100 = 0.
x(x + 50) -2(x + 50) = 0.
(x + 50) (x-2) = 0.
Saranya said:
9 years ago
(90/60)min = (36/(10 - x)) - (36/(10 + x));
How the minus comes between this?
How the minus comes between this?
Kalpana said:
9 years ago
Given.
(up_stream time) - (down_stream time) = 90mins.
36(miles)/(10-x) - 36/(10+x) = 90/60.
We get equation form of
x^2 + 48x - 100 = 0.
Hence, x = 2 and x = -50.
x = 2 is the answer.
(up_stream time) - (down_stream time) = 90mins.
36(miles)/(10-x) - 36/(10+x) = 90/60.
We get equation form of
x^2 + 48x - 100 = 0.
Hence, x = 2 and x = -50.
x = 2 is the answer.
Mariyada Ramesh said:
9 years ago
I too have the same doubt @Ram.
Please clear it.
Please clear it.
Raj said:
9 years ago
D=S/T or D=S * T @ Venkat.
Raj said:
9 years ago
@Venkat.
How
36=S/1.3
S= 2.
Explain this step.
How
36=S/1.3
S= 2.
Explain this step.
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)?
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.
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