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If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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13 Aug 2012, 07:09

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If Car X followed Car Y across a certain bridge that is 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.

Practice Questions Question: 16 Page: 276 Difficulty: 600

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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17 Aug 2012, 01:32

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SOLUTION

If Car X followed Car Y across a certain bridge that is 1/2 mile long, how many seconds did it take Car X to travel across the bridge?

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

(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. 30 miles per hour is \(\frac{30}{3600}=\frac{1}{120}\) miles per second --> car Y needs \(t_y=\frac{(\frac{1}{2})}{(\frac{1}{120})}=60\) seconds to travel across the bridge. Not sufficient to calculate \(t_x\).

(1)+(2) \(t_y=t_x+1\) and \(t_y=60\) --> \(t_x=60-1=59\). Sufficient.

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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16 Sep 2012, 21:43

Bunuel

Can this not be vvorked out just vvith A

--> since distances are equal then equate

SX. Tx = Sy. Ty

Is this not possible

then in that case statement 1 becomes sufficient please explain

Bunuel wrote:

SOLUTION

If Car X followed Car Y across a certain bridge that is 1/2 mile long, how many seconds did it take Car X to travel across the bridge?

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

(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. 30 miles per hour is \(\frac{30}{3600}=\frac{1}{120}\) miles per second --> car Y needs \(t_y=\frac{(\frac{1}{2})}{(\frac{1}{120})}=60\) seconds to travel across the bridge. Not sufficient to calculate \(t_x\).

(1)+(2) \(t_y=t_x+1\) and \(t_y=60\) --> \(t_x=60-1=59\). Sufficient.

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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03 May 2013, 16:43

In such questions, i look at the short concise option 1st. I look at B and it doesn't talk about one of the two cars. Eliminate answer choice B and D. Now look at A, it gives a relation between time. We can form two eqn for two cars using speed distance formulae. For 1st car assume all capital letters for its calculations. S= D/T For 2nd car , assume small letters: s=D/t

Now we know that t=T-1 since it took one sec less to travel the bridge. We also know the length of the bridge. However we don't know the speeds of time in absolute numbers to determine the answer. So we can't get anywhere with the 2Eqns.

Now add info from B and we can compute our answer. hence C.

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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07 May 2013, 14:05

I dont know if i am wrong but all i did was use the formula speed = distance x time and they have given us both speed ie the rate traveled and the time so am i wrong?

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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08 May 2013, 06:04

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madzstar wrote:

I dont know if i am wrong but all i did was use the formula speed = distance x time and they have given us both speed ie the rate traveled and the time so am i wrong?

The formula is (speed)*(time)=(distance). _________________

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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30 Jul 2013, 16:28

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If Car X followed Car Y across a certain bridge that is 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. We don't know how long Car Y was on the bridge for. Therefore, we don't know how long Car X was on the bridge for. INSUFFICIENT

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour. This tells us nothing about Car X. INSUFFICIENT

1+2) If the bridge is 1/2 mile long and Car Y crossed it at 30MPH it would have taken exactly one minute to cross the bridge. If Car X followed behind Car Y by 3 seconds and left the bridge just two seconds after Y it gained one second on Y. Therefore, Car X crossed the bridge in 29 seconds. SUFFICIENT

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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12 Jan 2014, 13:55

WholeLottaLove wrote:

If Car X followed Car Y across a certain bridge that is 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. We don't know how long Car Y was on the bridge for. Therefore, we don't know how long Car X was on the bridge for. INSUFFICIENT

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour. This tells us nothing about Car X. INSUFFICIENT

1+2) If the bridge is 1/2 mile long and Car Y crossed it at 30MPH it would have taken exactly one minute to cross the bridge. If Car X followed behind Car Y by 3 seconds and left the bridge just two seconds after Y it gained one second on Y. Therefore, Car X crossed the bridge in 29 seconds. SUFFICIENT

(C)

I think you mean 59 seconds. If it took one minute for Car Y to cross and Car X gained on Car Y by one second then the time it took would be 59 seconds.

Re: If Car X followed Car Y across a certain bridge that is 1/2 [#permalink]

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