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# If car X followed car Y across a certain bridge that is 21

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Joined: 21 May 2008
Posts: 85
If car X followed car Y across a certain bridge that is 21 [#permalink]

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09 Sep 2008, 06:16
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75% (00:40) correct 25% (00:00) wrong based on 10 sessions

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If car X followed car Y across a certain bridge that is 21 miles long, how many seconds did it take car X to travel across the bridge?

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.
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Joined: 07 Nov 2007
Posts: 1789
Location: New York

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09 Sep 2008, 06:28
klb15 wrote:
If car X followed car Y across a certain bridge that is 21 miles long, how many seconds did it take car X to travel across the bridge?

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.

(1) itself insufficient --> we need to know total time taken or average speed
(2) itself insufficient --> No info regarding X speed or time taken..

combined.

Y took the time = 21/30= 0.7 hr= 42*60 sec
X took the time = 42*60+1
suffcient.

C.
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Joined: 11 May 2008
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09 Sep 2008, 07:33
x2suresh wrote:
klb15 wrote:
If car X followed car Y across a certain bridge that is 21 miles long, how many seconds did it take car X to travel across the bridge?

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.

(1) itself insufficient --> we need to know total time taken or average speed
(2) itself insufficient --> No info regarding X speed or time taken..

combined.

Y took the time = 21/30= 0.7 hr= 42*60 sec
X took the time = 42*60+1should it not be lesser by 1 sec?????
suffcient.

C.

clearly both independently are insufficient.

together,
say 3 sec after first car entered, second car entered, and exited after 3 sec itself, then its speed is the same as the first one. its like having a car trailing with a iron rod long enough to enter after 3 seconds behind the leading car, and they both exit at the same time. then speed of both are same.
but here , the trailing car has gained 1 sec. so
i.e it has taken 1 sec less.
time of the second car= time of first car - 1 sec.

lovely problem , from where did u get this???
SVP
Joined: 07 Nov 2007
Posts: 1789
Location: New York

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09 Sep 2008, 07:37
arjtryarjtry wrote:
x2suresh wrote:
klb15 wrote:
If car X followed car Y across a certain bridge that is 21 miles long, how many seconds did it take car X to travel across the bridge?

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.

(1) itself insufficient --> we need to know total time taken or average speed
(2) itself insufficient --> No info regarding X speed or time taken..

combined.

Y took the time = 21/30= 0.7 hr= 42*60 sec
X took the time = 42*60+1should it not be lesser by 1 sec?????
suffcient.

C.

clearly both independently are insufficient.

together,
say 3 sec after first car entered, second car entered, and exited after 3 sec itself, then its speed is the same as the first one. its like having a car trailing with a iron rod long enough to enter after 3 seconds behind the leading car, and they both exit at the same time. then speed of both are same.
but here , the trailing car has gained 1 sec. so
i.e it has taken 1 sec less.
time of the second car= time of first car - 1 sec.

lovely problem , from where did u get this???

you caught me!!! luckily this is DS question..
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Smiling wins more friends than frowning

Current Student
Joined: 11 May 2008
Posts: 553

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09 Sep 2008, 07:39
u owe me a KF ..
u wanna join me for and

Re: DS-Across the Bridge   [#permalink] 09 Sep 2008, 07:39
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