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Catch up

by [email protected] » Mon Mar 08, 2010 10:13 pm
While on a straight road, car a and b are traveling at different constant rates. If car a is now 2 miles behind car b, how many minutes from now will car a catch up to car b?

1) Car a is traveling at 50 miles per hour and car b is traveling at 30 miles per hr
2) 3 mins ago car a was 3 miles behind car b
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Source: — Data Sufficiency |
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by papgust » Mon Mar 08, 2010 10:28 pm
Car A is 2 miles behind Car B. To know how long it would take to catch up, we need to know the speeds of car A and B.

i. Car A = 50 mph, Car B = 30 mph

So, Car A would catch up 20 miles in every hour/60 min. To catch up 2 miles, it will take 6 minutes for Car A. Sufficient.

ii. 3 minutes ago, Car A was 3 miles behind Car B.

From the question, we know that Car A is now 2 miles behind Car B. So, in every 3 minutes, Car A catch up 1 mile. To catch up 2 miles, it will take 6 minutes for Car A. Sufficient.

D
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by [email protected] » Mon Mar 08, 2010 11:29 pm
papgust,

How is II sufficient?
by the time A covers 2 mins in 6 mins, will B not cover (B's speed) X 6 mins?

To catch up, shouldn't A cover 2 mins as well as the distance that B covers in the mean time?
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by papgust » Mon Mar 08, 2010 11:47 pm
It isn't like that. Car A's and B's speeds are included already.

Whatever the speeds may be for Car A and B,
i. 3 min ago, Car A was 3 miles behind Car B

ii. Now, Car A is 2 miles behind Car B.

The gap b/w 2 cars gradually closes up. For every 3 min, gap of 1 mile is closed.

So, Car A needs 2 more miles to reach Car B. The gap of 1 mile closes every 3 min. So, in 6 min, the gap completely closes.
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