Speed problems

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gmatv09: Master | Next Rank: 500 Posts; Posts: 256; Joined: Mon Aug 10, 2009 6:31 pm; Thanked: 3 times

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

Source: Beat The GMAT — Problem Solving | Thu Dec 17, 2009 8:42 pm

satish.nagdev: Master | Next Rank: 500 Posts; Posts: 130; Joined: Sat Dec 29, 2007 5:35 am; Location: SGP; Thanked: 5 times

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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batbond007: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Tue Sep 29, 2009 2:03 pm

by batbond007 » Fri Dec 18, 2009 9:53 am

Ans E i.e 100 meters for the first one.

satish.nagdev: Master | Next Rank: 500 Posts; Posts: 130; Joined: Sat Dec 29, 2007 5:35 am; Location: SGP; Thanked: 5 times

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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regor60: Master | Next Rank: 500 Posts; Posts: 416; Joined: Thu Oct 15, 2009 11:52 am; Thanked: 27 times

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

djkvakin: Senior | Next Rank: 100 Posts; Posts: 84; Joined: Sat Oct 03, 2009 1:13 pm; Thanked: 11 times

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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