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 5 of 9.
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.
Akash said:
6 years ago
@Siddesh.
Boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream.
It means while going downstream the time will be less and will while going upstream (i.e against) it will take 90 min more;
so t=d/s.
Boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream.
It means while going downstream the time will be less and will while going upstream (i.e against) it will take 90 min more;
so t=d/s.
Guna de villiers said:
6 years ago
D/S = T.
Time difference is 90.
We have to convert miles per HOUR.
So (36/10-x)-(36/10+x) = 90/60.
Time difference is 90.
We have to convert miles per HOUR.
So (36/10-x)-(36/10+x) = 90/60.
Joan said:
1 decade ago
Please explain how you have got (x+50) (x-2).
Abhinn said:
1 decade ago
Please explain what does 90 min less means?
Sowmya said:
1 decade ago
Take 36 common,
36(1/(10-x)-1/(10+x)) = 90/60.
36((10+x)-(10-x))/(100-x2) = 3/2 since in Denominator is in the form (a+b)(a-b) = a2-b2.
And 90/60 = 2/3.
36((10+x-10+x)/100-x2 = 3/2.
36(2x/100-x2) = 3/2.
72x/100-x2 = 3/2.
hence 72x*2 = 3(100-x2).
3x2+144x-300 = 0.
x2+48x-100 = 0.
(x+ 50)(x - 2) = 0.
x = 2 mph.
Hope you understand guys.
36(1/(10-x)-1/(10+x)) = 90/60.
36((10+x)-(10-x))/(100-x2) = 3/2 since in Denominator is in the form (a+b)(a-b) = a2-b2.
And 90/60 = 2/3.
36((10+x-10+x)/100-x2 = 3/2.
36(2x/100-x2) = 3/2.
72x/100-x2 = 3/2.
hence 72x*2 = 3(100-x2).
3x2+144x-300 = 0.
x2+48x-100 = 0.
(x+ 50)(x - 2) = 0.
x = 2 mph.
Hope you understand guys.
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?
Puneeth kumar k said:
1 decade ago
Because in question they have mentioned that 90min less.
Sandeep said:
1 decade ago
In 4th problem '+' is used why '-' hear.
36/(10 - x) "-" 36/(10 + x) = 90/60.
36/(10 - x) "-" 36/(10 + x) = 90/60.
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.
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