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Manager
Joined: 08 Dec 2006
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A swimmer makes a round trip up and down the river. It takes him X hours. The next day he swims the same distance with the same speed in the still water. It takes him Y hours. What can we say ABOUT X and Y ?
1. X>Y
2.X<Y
3.X=Y
4. X= 1/2 Y
5. NONE OF THE ABOVE
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Abhishek.pitti wrote: A swimmer makes a round trip up and down the river. It takes him X hours. The next day he swims the same distance with the same speed in the still water. It takes him Y hours. What can we say ABOUT X and Y ?
1. X>Y
2.X<Y
3.X=Y
4. X= 1/2 Y
5. NONE OF THE ABOVE I assumed that "the same distance" is different from " a round trip", so a is the choice
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Abhishek.pitti wrote: A swimmer makes a round trip up and down the river. It takes him X hours. The next day he swims the same distance with the same speed in the still water. It takes him Y hours. What can we say ABOUT X and Y ?
1. X>Y
2.X<Y
3.X=Y
4. X= 1/2 Y
5. NONE OF THE ABOVE Time = Distance travelled/speed assume v= speead in the still water s = stream speed D = Distance X = time taken for upstream +time taken for downstream = D/(v-s) + D/(v+s) = D(2v)/(v^2-s^2) = 2D/(v-(s^2)/v) Y = 2D/v from the above clearly X>Y
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Manager
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Distance = D
speed in upstream S-a, speed in downstream S+a
speed in still water = S
for Still water, for both up and down,
So 2D = S/Y
Y = S/(2D) ----- a)
For upstream and downstream,
So D = S-a/t1(upstream )
D = S+a/t2 (downstream)
Dt1 = S-a --- 1
Dt2 = S+a ----2
Adding 1) and 2) t1+t2 = X
DX = 2S ===> X=2S/D ---- b)
from a) and b)
x=4D
S0 X>Y
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x2suresh wrote: Abhishek.pitti wrote: A swimmer makes a round trip up and down the river. It takes him X hours. The next day he swims the same distance with the same speed in the still water. It takes him Y hours. What can we say ABOUT X and Y ?
1. X>Y
2.X<Y
3.X=Y
4. X= 1/2 Y
5. NONE OF THE ABOVE Time = Distance travelled/speed assume v= speead in the still water s = stream speed D = Distance X = time taken for upstream +time taken for downstream = D/(v-s) + D/(v+s) = D(2v)/(v^2-s^2) = 2D/(v-(s^2)/v) Y = 2D/v from the above clearly X>Y I missed the conversion below and was wondering how to attack the problem. = 2D/(v-(s^2)/v) Y = 2D/v Thank you. 
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Another approach Let's imagine we slowly change speed of stream from 0 to speed of a swimmer: the time change from Y to Infinity (the swimmer cannot swim against a strong stream). As speed of stream is greater than 0 - therefore, X>Y Sometimes another approach could help when we suddenly forget necessary formulas or have no time to write out something... 
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walker wrote: Another approach Let's imagine we slowly change speed of stream from 0 to speed of a swimmer: the time change from Y to Infinity (the swimmer cannot swim against a strong stream). As speed of stream is greater than 0 - therefore, X>Y Sometimes another approach could help when we suddenly forget necessary formulas or have no time to write out something... you are machine... great approach!!!!!
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walker wrote: Another approach Let's imagine we slowly change speed of stream from 0 to speed of a swimmer: the time change from Y to Infinity (the swimmer cannot swim against a strong stream). As speed of stream is greater than 0 - therefore, X>Y Sometimes another approach could help when we suddenly forget necessary formulas or have no time to write out something... Excellent explanation! But, this can be understood only for swimming against the stream. In round trip, initial understanding comes as the time lost against the stream will equal the time gained along the stream and hence total time will be the same.
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In my approach I take two edge points: 1) speed of stream = speed of swimmer ---> Y=X 2) speed of stream = speed of swimmer ---> Y=infinity >> X Now, I assume that Y changes gradually from X to infinity between these points. I agree with you that it is an assumption and there is no any "special" point in the interval. At the same time I see the assumption pretty natural as at point 2 downstream time decreases only by two times (relative speed of a swimmer doubles) but upstream time increases by infinite times (relative speed of a swimmer is zero). In other words, downstream time decreases much slowly than upstream time increases.
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walker wrote: In my approach I take two edge points:
1) speed of stream = speed of swimmer ---> Y=X
2) speed of stream = speed of swimmer ---> Y=infinity >> X
Now, I assume that Y changes gradually from X to infinity between these points. I agree with you that it is an assumption and there is no any "special" point in the interval. At the same time I see the assumption pretty natural as at point 2 downstream time decreases only by two times (relative speed of a swimmer doubles) but upstream time increases by infinite times (relative speed of a swimmer is zero). In other words, downstream time decreases much slowly than upstream time increases. crystal clear. Thanks walker!
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Manager
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scthakur wrote: walker wrote: In my approach I take two edge points:
1) speed of stream = speed of swimmer ---> Y=X
2) speed of stream = speed of swimmer ---> Y=infinity >> X
Now, I assume that Y changes gradually from X to infinity between these points. I agree with you that it is an assumption and there is no any "special" point in the interval. At the same time I see the assumption pretty natural as at point 2 downstream time decreases only by two times (relative speed of a swimmer doubles) but upstream time increases by infinite times (relative speed of a swimmer is zero). In other words, downstream time decreases much slowly than upstream time increases. crystal clear. Thanks walker! Thakur, can you put Walker's explanation in some other words? I don't get it. I read it 3-4 times but just don't get it. Just wondering if this can be presented in alternate way?
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CEO
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Perhaps, it is because a misprint. It should be: 1) speed of stream = 0 ---> Y=X 2) speed of stream = speed of swimmer ---> Y=infinity >> X
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suresh, walker, thanks for explanations. +2
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Manager
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I still dont get it. I think the answer is 'C' - X=Y.
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Ibodullo wrote: I still dont get it. I think the answer is 'C' - X=Y. Time = Distance travelled/speed assume v= speead in the still water s = stream speed D = Distance X = time taken for upstream +time taken for downstream = D/(v-s) + D/(v+s) = D(2v)/(v^2-s^2) = 2D/(v-(s^2)/v) = 2D/(v-k) { say s^2/v = k, which is always postive unless stream speed =0 (in this case k=0)} Y = 2D/v from the above clearly X>Y (because v-k is always less than v } Did you get it now?
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x2suresh wrote: Ibodullo wrote: I still dont get it. I think the answer is 'C' - X=Y. Time = Distance travelled/speed assume v= speead in the still water s = stream speed D = Distance X = time taken for upstream +time taken for downstream = D/(v-s) + D/(v+s) = D(2v)/(v^2-s^2) = 2D/(v-(s^2)/v) = 2D/(v-k) { say s^2/v = k, which is always postive unless stream speed =0 (in this case k=0)} Y = 2D/v from the above clearly X>Y (because v-k is always less than v } Did you get it now? Understood and agreed. Thanks a lot Suresh!
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