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Speed problems

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Speed problems

by gmatv09 » Thu Dec 17, 2009 8:42 pm
Find out the combined length of the trains if one traveling at 40 km per hour and crosses
another train traveling in same direction at 30 miles per hour in 20 seconds.

a. 150 meters
b. 200 meters
c. 200/3 meters
d. 500/9 meters
e. 100 meters


Raffela walks a certain distance and rides back taking a total time of 50 minutes.
She could walk both ways in 70 minutes. How long would it take him to ride both ways?

a. 40 minutes
b. 100 minutes
c. 120 minutes
d. 25 minutes
e. 30 minutes


A train covers a distance of 20 km in 15 minutes. If its speed is decreased by 20km/hr,
what is the time taken by it to cover the same distance?
a. 10 minutes
b. 15 minutes
c. 20 minutes
d. 40 minutes
e. 80 minutes
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by satish.nagdev » Fri Dec 18, 2009 12:01 am
for second
say x = walking speed and y be riding speed and d be one side distance

d = s * t so
d/x + d/y = 50 minutes (I)
2d/x=70 so d/x = 35

putting in first d/y + 35 = 50 so one side d/y = 15 which is one side ride hence 30minutes for both ways


in 3rd
20km is covered in 15 minutes so 80km/hr is the speed, this is reduced by 20 that makes it to 60km/hr now to cover 20km at this speed 20 minutes are needed.

1st one is bit confusing :(
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by batbond007 » Fri Dec 18, 2009 9:53 am
Ans E i.e 100 meters for the first one.
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by satish.nagdev » Sat Dec 19, 2009 5:25 am
batbond007 wrote:Ans E i.e 100 meters for the first one.
can you explain how did you get that answer?
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by regor60 » Mon Dec 21, 2009 7:31 am
Answer for question 1 is "D"

You have a units conflict...30 mph must be 30kph

Same direction > 10 kph relative speed..one train not moving, the other moving at 10 kph

[(10,000 meters/hr)*20 seconds]/ 3600 seconds/hr = 55.55 meters = 500/9 meters
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by djkvakin » Mon Dec 21, 2009 9:13 am
regor60 wrote:Answer for question 1 is "D"

You have a units conflict...30 mph must be 30kph

Same direction > 10 kph relative speed..one train not moving, the other moving at 10 kph

[(10,000 meters/hr)*20 seconds]/ 3600 seconds/hr = 55.55 meters = 500/9 meters
I agree that the length of the train going 30km/h is 500/9, but the question asks about the combined length of 2 trains. I am not sure how to solve this. Anyone? it doesn't say they have equal length, does it?
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