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Trains

by Aman verma » Sun May 23, 2010 8:50 am
Q: By what percentage is the speed of the faster train more than that of slower train ?

I. Length of the slower train is thrice that of the faster train and the distance between the rear ends is 400 mts when their front ends are at the same point .

II.The ratio of times taken by the faster train to cross the slower train, when they are travelling in the opposite direction and when they are travelling in the same direction is 3 : 5
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by liferocks » Sun May 23, 2010 9:04 am
From 1,
we can calculate the length of each train but no information about the time taken to cross each other or the speed of the trains...insufficient

From 2,
Since distance is same when the faster trains crosses the slower train in same and opposite direction(sum of length of the trains),ratio of relative speed will be inversely equal to ratio of time taken.
Let speed of faster and slower train be s1 and s2 respectively
hence (s1-s2)/(s1+s2)=3/5
or s1/s2=8/2 or s1=4s2

so the speed of the faster train is 300% more than that of slower train...sufficient

Ans option B
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by Gmat Bond » Fri May 28, 2010 1:19 am
What's the OA ?
The name is Bond, GMAT BOND !!
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by indiantiger » Sat May 29, 2010 3:27 pm
Need to find the % greater

Given : one train is slow and one is fast

Lets say slow trains parameters are Ls of length and s for speed
for fast train parameters are Lf for length and F for speed.

1) Ls> Lf or Ls = 3 *Lf
Ls -Lf = 400
does not give us any thing about speed so insufficient
2) t = d/s

now in this case there are two cases

i) trains are moving in same direction so relative speed is (f -s)

t1 = d /(f-s)

ii)trains are moving in oppst direction so relative speed is(f+s)
t2 = d/(f+s)

we are given the ratio of time when they are travelling in the opposite direction and when they are travelling in the same direction is 3 : 5

t2/t1 = 3/5

as distance is same

(1/f+s)/(1/f-s) = 3/5
f-s/f+s = 3/5
=> f =4s
(f-s)/s => 3s/s *100 = 300% (B) answer
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by Aman verma » Mon May 31, 2010 11:35 pm
Many thanks to all ! I don't have the answer or the explanation , but it's definitely not C,D or E. It's either A or B.
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by Pdgmat2010 » Sun Jul 18, 2010 11:01 pm
isn't it b as per liferocks explanation?

one thing i didnt get : how is the distance travelled the same whether the trains are going in same or opposite directions?
someone care to explain?

many thanks!

cheers,
pd
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by sumanr84 » Mon Jul 19, 2010 12:50 am
if x is the length of one train and y is the length of other train.
Then to fully cross each other would require both trains to cross the entire length (x+y) no matter whichever direction they are traveling.

Just visualize and it will be clear.
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