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2 speed problems

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by GmatTakerNo.1 » Sun May 02, 2010 1:39 pm
Hey, can you help me with these questions?

1. A train passes a man standing on a platform in 9 sec. and passes the platform in 12 sec. completely. If the platform is 50 meters long, how long is the train?

Answer: 150m

2. If train traveling at 80 km/h crosses a bridge of a length of 750 meters completely in 45 seconds, what is the length of the train?


Answer: 250m
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by clock60 » Sun May 02, 2010 1:58 pm
as for problem 2
let it be x the lengh of the train, then the distance covered in a 45 sec=1/80 hour, is 750 m=3/4 km+x
(3/4+x)/80=1/80
x=1/4
1/4*1000=250
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by clock60 » Sun May 02, 2010 3:04 pm
for problem1
let it be x the lengh of the train, and v-train speed

(50/1000+x)/v=12/3600 -this for passing the train near platform

x/v=9/3600 passing the train near the man

solving for x=15/100*1000=150
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by liferocks » Sun May 02, 2010 6:12 pm
We can avoid forming the equation in this way

1. the train crosses own length ==9s
crosses own len+platform=12s
crosses platform =12-9=3s
hence in 3 sec it crosses 50m
length of thrai=(9/3)*50=150m(Ans)

2.speed of train is 80km/h
in 1/80h it goes=1km=1000m
length of train 1000-750=250m(Ans)
"If you don't know where you are going, any road will get you there."
Lewis Carroll
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by ssuarezo » Mon May 03, 2010 12:39 pm
liferocks wrote:We can avoid forming the equation in this way

2.speed of train is 80km/h
in 1/80h it goes=1km=1000m
length of train 1000-750=250m(Ans)
Hi Liferocks:

Why 1000-750? at 1/80 it makes 1000m, ok, but you're not using the 45 seconds given as info. besides, I see no relation in 1/80 km/h and 750 m.

Thanks for clarifying your answer.
Silvia
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by this_time_i_will » Mon May 03, 2010 5:25 pm
GmatTakerNo.1 wrote:Hey, can you help me with these questions?

1. A train passes a man standing on a platform in 9 sec. and passes the platform in 12 sec. completely. If the platform is 50 meters long, how long is the train?

Answer: 150m

2. If train traveling at 80 km/h crosses a bridge of a length of 750 meters completely in 45 seconds, what is the length of the train?


Answer: 250m
For problem II:
Use the simple formulae of Distance = speed*time
Let the lenght of train is x km. (yes putting in km would be easier for calc.)
.75+x = 80*1/80
so x = 1-.75 = .25KM or 250 m.
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by liferocks » Mon May 03, 2010 7:31 pm
ssuarezo wrote:
liferocks wrote:We can avoid forming the equation in this way

2.speed of train is 80km/h
in 1/80h it goes=1km=1000m
length of train 1000-750=250m(Ans)
Hi Liferocks:

Why 1000-750? at 1/80 it makes 1000m, ok, but you're not using the 45 seconds given as info. besides, I see no relation in 1/80 km/h and 750 m.

Thanks for clarifying your answer.
Silvia
the train crosses the bridge of length 750m in 45sec=45*1/60*60 hr=1/80hr

hope this clarifies the confusion.
"If you don't know where you are going, any road will get you there."
Lewis Carroll
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by frank1 » Mon May 03, 2010 9:02 pm
liferocks wrote:
ssuarezo wrote:
liferocks wrote:We can avoid forming the equation in this way

the train crosses the bridge of length 750m in 45sec=45*1/60*60 hr=1/80hr

hope this clarifies the confusion.
i still could not see where 1/80 is used in answer process....
infact i am wondering why answer is correct here....(testing difference cases)
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by liferocks » Mon May 03, 2010 9:19 pm
frank1 wrote:
liferocks wrote:
ssuarezo wrote:
liferocks wrote:We can avoid forming the equation in this way

the train crosses the bridge of length 750m in 45sec=45*1/60*60 hr=1/80hr

hope this clarifies the confusion.
i still could not see where 1/80 is used in answer process....
infact i am wondering why answer is correct here....(testing difference cases)
The question is If train traveling at 80 km/h crosses a bridge of a length of 750 meters completely in 45 seconds what is the length of the train.

when a train crosses a bridge it crosses bridge lenght + train length

so train crosses bridge length +train length in 45s .
45s=1/80hr
speed of train is 80km/hr
hence in 1/80hr it will cross 80*1/80=1km=1000m

so bridge length +train length=1000 m
length of bridge 750 meters
so length of train is 1000-750=250m
"If you don't know where you are going, any road will get you there."
Lewis Carroll
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by nakul_anand » Tue May 04, 2010 2:38 am
'A train passes a man standing on a platform in 9 seconds and passes the platform in 12 seconds completely'.

How do we know that the train covers a distance equivalent to its own length in 9 seconds?
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by nakul_anand » Tue May 04, 2010 2:39 am
'A train passes a man standing on a platform in 9 seconds and passes the platform in 12 seconds completely'.

How do we know that the train covers a distance equivalent to its own length in 9 seconds?
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by ssuarezo » Tue May 04, 2010 6:09 am
liferocks wrote:
ssuarezo wrote: Hi Liferocks:

Why 1000-750? at 1/80 it makes 1000m, ok, but you're not using the 45 seconds given as info. besides, I see no relation in 1/80 km/h and 750 m.

Thanks for clarifying your answer.
Silvia
the train crosses the bridge of length 750m in 45sec=45*1/60*60 hr=1/80hr

hope this clarifies the confusion.
Hi Liferocks:

True ! .. as simple as converting 45 seconds to hours: 45sec/3600 sec = 1/80 hr., and in that time, he makes 1000m, but we have to substract 750, the distance of the bridge, already considered in the 45 seconds, so we have 250m as the length of the train.
Thanks for your patience,
Silvia
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by ssuarezo » Tue May 04, 2010 6:18 am
nakul_anand wrote:'A train passes a man standing on a platform in 9 seconds and passes the platform in 12 seconds completely'.

How do we know that the train covers a distance equivalent to its own length in 9 seconds?
Nakul:
The questions says "A train passes a man standing on a platform", when it says is passes, it means it takes 9 seconds since the train first reach the man until the train passed this man. No matter where in the platform he's standing in. Same thing for the platform, it takes 12 seconds to pass it completely, but in this case you have the lenght of the train + the length of the platform included in those 12 seconds.

hoep this helps
Silvia
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