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.
Rishek said:
3 years ago
Let's take 2.
Down stream(time taken) - 36/(10+2)= 3hrs.
Up stream (time taken) - 36/(10-2) = 4.5hrs.
Difference = 1.5hrs which is =90min.
So, option A is correct.
Down stream(time taken) - 36/(10+2)= 3hrs.
Up stream (time taken) - 36/(10-2) = 4.5hrs.
Difference = 1.5hrs which is =90min.
So, option A is correct.
(37)
Ravi Kumar said:
1 decade ago
After getting X+50 and X-2 as factors of the quadratic equation, why two has been considered as answer and why not 50?
Considering X=2 is not clear. Please explain.
Considering X=2 is not clear. Please explain.
Shubh said:
1 decade ago
Sorry but I am not able to understand as to how the equation 72x x 60 = 90 (100 - x2) is reduced into x2 + 48x + 100 = 0.
Could you explain in more detail?
Could you explain in more detail?
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)?
Gopal said:
1 decade ago
Downstream-upstream = ((36/(10+x)) - ((36/(10-x)) ;
I have tried with this I got clarification if any one has any doubt check with this.
Thank you.
I have tried with this I got clarification if any one has any doubt check with this.
Thank you.
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.
Guru said:
1 decade ago
Two roots of eqn are 2 and -50.
We can't take -ve root so 2 is the answer.
Hence the equation (x+ 50)(x - 2) = 0 from x2+ 48x + 100 = 0.
We can't take -ve root so 2 is the answer.
Hence the equation (x+ 50)(x - 2) = 0 from x2+ 48x + 100 = 0.
Siddesh said:
6 years ago
Can anyone say please, how can we equate both the upstream and downstream to 90, if downstream takes less than 90 minutes? Please tell me.
Naveen said:
7 years ago
If we put x=2 in 36/(10-x)-36/(10+x) = 90/60,
We get 1=3/2.
Which is invalid, then how can we say that x= 2?
Anyone can tell me?
We get 1=3/2.
Which is invalid, then how can we say that x= 2?
Anyone can tell me?
Prem said:
5 years ago
Let upstream time be T hrs.
Then, downstream time will be T-3/2 hrs.
Upstream - downstream = T - T + 3/2 = 3/2 hrs = 90 minutes.
Then, downstream time will be T-3/2 hrs.
Upstream - downstream = T - T + 3/2 = 3/2 hrs = 90 minutes.
(1)
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