Rate and Distance

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preciousrain7: Senior | Next Rank: 100 Posts; Posts: 78; Joined: Thu Dec 06, 2007 4:16 pm; Location: NYC, NY; Thanked: 2 times

by preciousrain7 » Wed Jan 16, 2008 5:04 pm

If someone can give me an overview of how to calculate collision problems, or when the "two people met problems" or "after how many miles did the two people meet", i'd really appreciate it!!!

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Source: Beat The GMAT — Problem Solving | Wed Jan 16, 2008 5:04 pm

Suyog: Master | Next Rank: 500 Posts; Posts: 315; Joined: Thu Aug 17, 2006 10:43 pm; Thanked: 23 times

by Suyog » Wed Jan 16, 2008 5:46 pm

Here the total distance is 75 miles from Fran junction to Thomasville.

1. A travel at speed of 50 miles per hour and B travels at 25 miles per hour.

after an hour A will be 50 miles from Fran Junction and B will be 25 miles from Thomasville. So adding both gives 75, which is the distance between the 2 stations. Hence the trains will be 25 miles away from Thomasville when they collide. Suff

2. Gives only data that says trains collide after an hour, it does not give any information about the speed or the distance travelled and hence we can not calculate how far they will be from Thomasville. Insuff

Ans A.

While answering collide/collusion related problems dont assume any numbers. Just work with the information provided. Also calculate the numbers from both the ways... i.e. in terms of the above example.... calculate the distance from A at the speed given irrespective of B and vice-versa.
When there are more than 2 numbers involved... can be in terms of problem solving... try to work with the ans choices...it will help u eliminate the ans choices very fast...

Hope it helps...

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preciousrain7: Senior | Next Rank: 100 Posts; Posts: 78; Joined: Thu Dec 06, 2007 4:16 pm; Location: NYC, NY; Thanked: 2 times

by preciousrain7 » Wed Jan 16, 2008 6:02 pm

Suyog wrote:Here the total distance is 75 miles from Fran junction to Thomasville.

1. A travel at speed of 50 miles per hour and B travels at 25 miles per hour.

after an hour A will be 50 miles from Fran Junction and B will be 25 miles from Thomasville. So adding both gives 75, which is the distance between the 2 stations. Hence the trains will be 25 miles away from Thomasville when they collide. Suff

2. Gives only data that says trains collide after an hour, it does not give any information about the speed or the distance travelled and hence we can not calculate how far they will be from Thomasville. Insuff

Ans A.

While answering collide/collusion related problems dont assume any numbers. Just work with the information provided. Also calculate the numbers from both the ways... i.e. in terms of the above example.... calculate the distance from A at the speed given irrespective of B and vice-versa.
When there are more than 2 numbers involved... can be in terms of problem solving... try to work with the ans choices...it will help u eliminate the ans choices very fast...

Hope it helps...

THANKS SO MUCH! It does help a lotttt

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kaplansteve: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Tue Jan 15, 2008 12:15 pm

by kaplansteve » Thu Jan 17, 2008 12:00 pm

The simple equation I think you're looking for here is just to add the rates. So if train 1 is going 50 mph and train 2 is going half that, together they're going 75 mph. From that you can find the time they collide (d/r=t, 75/75=1 hour) and solve everything else from there. Adding the rates is an easy way to solve if things are moving towards or away from each other.

If one thing is catching up to another, instead subtract the rates to find out how long it takes one to reach the other.

Cheers,
Steve

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preciousrain7: Senior | Next Rank: 100 Posts; Posts: 78; Joined: Thu Dec 06, 2007 4:16 pm; Location: NYC, NY; Thanked: 2 times

by preciousrain7 » Thu Jan 17, 2008 2:31 pm

kaplansteve wrote:The simple equation I think you're looking for here is just to add the rates. So if train 1 is going 50 mph and train 2 is going half that, together they're going 75 mph. From that you can find the time they collide (d/r=t, 75/75=1 hour) and solve everything else from there. Adding the rates is an easy way to solve if things are moving towards or away from each other.

If one thing is catching up to another, instead subtract the rates to find out how long it takes one to reach the other.

Cheers,
Steve

Oh this is great! I will def remember that for the test.

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