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600-700 level rates problm

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by rtaha2412 » Wed Dec 15, 2010 12:08 pm
I am having trouble understanding and recognizing relative rates problems and when to subtract or add rates. Please advise

While on a straight road, Car X and Car Y are traveling
at different constant rates. If Car X is now 1 mile
ahead of Car Y, how many minutes from now will
Car X be 2 miles ahead of Car Y ?

(1) Car X is traveling at 50 miles per hour and Car Y
is traveling at 40 miles per hour.

(2) Three minutes ago Car X was 1/2
mile ahead of
Car Y.
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Source: — Data Sufficiency |
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by HPengineer » Wed Dec 15, 2010 12:36 pm
I belive the keys to this problem is understanding that X is moving ahead of y by a speed of (x-y)

So x = 50 and y = 40 then X is driving away from X at 10 mph. If he is currently 1 mile ahead then he only needs 1mile/10mph i belive that is the time he will be 2 miles ahead..

I belive same concept for statement 2 since x is gaining on y at 1/2 every 3 min then it should take him six minutes to travel a mile...

By my logic.. not sure if its right or not i would select ans D..
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by tomada » Wed Dec 15, 2010 1:15 pm
I agree with HPengineer. The answer is (D).

HPengineer wrote:I belive the keys to this problem is understanding that X is moving ahead of y by a speed of (x-y)

So x = 50 and y = 40 then X is driving away from X at 10 mph. If he is currently 1 mile ahead then he only needs 1mile/10mph i belive that is the time he will be 2 miles ahead..

I belive same concept for statement 2 since x is gaining on y at 1/2 every 3 min then it should take him six minutes to travel a mile...

By my logic.. not sure if its right or not i would select ans D..
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by GMATGuruNY » Thu Dec 16, 2010 4:39 am
rtaha2412 wrote:I am having trouble understanding and recognizing relative rates problems and when to subtract or add rates. Please advise

While on a straight road, Car X and Car Y are traveling
at different constant rates. If Car X is now 1 mile
ahead of Car Y, how many minutes from now will
Car X be 2 miles ahead of Car Y ?

(1) Car X is traveling at 50 miles per hour and Car Y
is traveling at 40 miles per hour.

(2) Three minutes ago Car X was 1/2
mile ahead of
Car Y.
Since Car X is now 1 mile ahead, and we're being asked how many minutes from now Car X will be 2 miles ahead, we need to know how long it will take Car X to travel 1 more mile than Car Y. To answer this question, we need to know the relative rates for X and Y.

Statement 1:
Tells us the rates for both cars, so we can calculate how much time will be needed for Car X to travel 1 more mile than Car Y. Sufficient. (Do not calculate the time!)

Statement 2:
3 minutes ago Car X was 1/2 mile ahead. Now -- 3 minutes later -- Car X is 1 mile ahead. So every 3 minutes, Car X travels 1/2 mile more than Car Y. Thus Car X will need 6 minutes to travel 1 more mile than Car Y. Sufficient.

The correct answer is D.
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