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If car X followed car Y across a certain bridge that is 21-m
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Author:  niheil [ 09 Oct 2010, 12:13 ]
Post subject:  If car X followed car Y across a certain bridge that is 21-m

If car X followed car Y across a certain bridge that is 21-mile long, how many seconds did it take car X to travel across the bridge?

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.
(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.

Additional info on the problem
Source: Paper Test
Test Code: 42
Section: 2 (Data Sufficiency)
Problem: 5

Author:  Bunuel [ 09 Oct 2010, 12:43 ]
Post subject:  Re: Plz Help with Data Sufficiency problem

niheil wrote:
Hi guys,

Please explain how to answer the following Data Sufficiency question:


If car X followed car Y across a certain bridge that is 21mile long, how many seconds did it take car X to travel across the bridge?
(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge.
(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.


Let the time needed for car X to travel across the bridge be \(t_x\) seconds and the time for Y \(t_y\) seconds.

Question: \(t_x=?\)

(1) Car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge --> car X needs 1 second less to travel across the bridge than car Y --> \(t_y=t_x+1\). Not sufficient to calculate \(t_x\).

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour = \(\frac{30}{3600}=\frac{1}{120}\) miles per second --> car Y needs \(t_y=\frac{21}{\frac{1}{120}}=21*120\) seconds to travel across the bridge. Not sufficient to calculate \(t_x\).

(1)+(2) \(t_y=t_x+1\) and \(t_y=21*120\) --> \(t_x=21*120-1\). Sufficient.

Answer: C.

Author:  niheil [ 09 Oct 2010, 14:00 ]
Post subject:  Re: Plz Help with Data Sufficiency problem

Awesome! Thanks again, Bunuel. I wish you could take the GMAT for me, lol.

Author:  PennState08 [ 02 Dec 2010, 14:09 ]
Post subject:  Rate Problem

All,
This was a data sufficiency problem, but want to double check my math in case I see it in a problem solver.
If Car X followed Car Y across a certain bridge that is \(\frac{1}{2}\) mile long, how many seconds did it take Car X to travel across the bridge?
(1) Car X drove onto the bridge exactly 3 seconds after Car Y drove onto the bridge and drove off the bridge exactly 2 seconds after Car Y drove off the bridge.
(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour.
C is the correct answer for data sufficiency, but I want to go through the problem in various ways (problem solving) to make sure my math is correct.

Provided all the data; What are the rates of each Car? How many seconds would it take for Car X to catch Car Y? At what distance would Car X catch Car Y?
Rate Car X: ~30.5 mph OR \(\frac{1}{118}\) miles per second
Rate Car Y: 30 mph (given) OR \(\frac{1}{120}\) miles per second
Second for Car X to catch Car Y: 180 seconds OR 3 minutes OR \(\frac{1}{20}\) hour
Distance: 1.5 miles

Explanations:
Rate x Time(t) = Distance
Car Y (given at 30 mph, so find \(t\) to solve for rate of car X)
\(y\) x \(t\) = \(\frac{1}{2}\)
30 x \(t\) = \(\frac{1}{2}\) (need miles per second, not hour)
\(\frac{1}{120}\) x \(t\) = \(\frac{1}{2}\)
\(t\) = 60
Car X
\(x\) x (\(t\) - 1) = \(\frac{1}{2}\) ; 1 second for the time difference (waited 3 seconds after Y, finished 2 seconds after Y: 3 - 2 = 1)
\(x\) x (60 - 1) = \(\frac{1}{2}\)
\(x\) x 59 = \(\frac{1}{2}\)
\(x\) = \(\frac{1}{118}\)

Time (in seconds) Car X catches Car Y:
\(\frac{1}{118}\) (\(t\) - 3) = \(\frac{1}{120}t\) ; 3 = amount of seconds after Y left.
\(\frac{1}{118} t\) - \(\frac{3}{118}\) = \(\frac{1}{120}t\)
\(\frac{60}{7080}t\) - \(\frac{180}{7080}\) = \(\frac{59}{7080}\)
\(\frac{1}{7080} t\) = \(\frac{180}{7080}\)
\(t\) = 180 seconds
180 seconds for Car X to catch Car Y

Distance (in miles) for Car X to catch Car Y
Taking either equation and substituting 180 for \(t\)
X) \(\frac{1}{118}\) x (180-3) = 1.5 miles
Y) \(\frac{1}{120}\) x (180) = 1.5 miles

I have all this set up correctly, right?

Author:  Bunuel [ 02 Dec 2010, 14:20 ]
Post subject:  Re: Rate Problem

Merging similar topics. The only difference is in the length of the bridge (21 miles in first question and 1/2 miles in the second one). But the answer for both of them is C. Please ask if anything remains unclear.

Author:  Basshead [ 13 Oct 2020, 09:37 ]
Post subject:  Re: If car X followed car Y across a certain bridge that is 21-m

(1) This tells us Car X crosses the bridge 1 second faster than Car Y. We can't determine the time it took for Car X to cross the bridge.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour. We can determine the time it takes Car Y to cross the bridge; however, this does not tell us anything about Car X

(1 & 2) From Statement 2 we can determine the time it takes Car Y to cross the bridge. We know Car X crosses the bridge 1 second faster than Car Y. Therefore, with both statements, we can determine the time it takes Car X to cross the bridge.

Author:  Hoozan [ 25 May 2021, 00:52 ]
Post subject:  Re: If car X followed car Y across a certain bridge that is 21-m

EducationAisle based on (1) doesn't car X take 5 seconds longer that car Y? Why is this not correct?

Author:  EducationAisle [ 25 May 2021, 02:52 ]
Post subject:  Re: If car X followed car Y across a certain bridge that is 21-m

Hoozan wrote:
EducationAisle based on (1) doesn't car X take 5 seconds longer that car Y? Why is this not correct?

Not really 5 seconds Hoozan.

Car X
a) drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and
b) drove off the bridge exactly 2 seconds after car Y drove off the bridge.

Notice that if X and Y had same speeds, then X would have driven off the bridge exactly 3 seconds after car Y drove off the bridge (because X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge).

However, since X drove drove off the bridge exactly 2 seconds after car Y drove off the bridge, this would mean that X "caught up" with Y, 1 second, on the bridge. So, car X actually took 1 second less than car Y, to travel the bridge.

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Post subject:  Re: If car X followed car Y across a certain bridge that is 21-m

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