If Car X followed Car Y across a certain bridge that is 1/2

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

The question asks the time taken by X to cross the bridge.

How come? Statement 1 helps us to get the relationship between X and Y wrt time. It does not tell us the time taken by Y to cross

Statement 2 gives us the time taken by Y to cross the bridge.

Both the statements together are sufficient.

Hope this helps!


Question level - 600

Banuel,

I'm not seeing it, is there a way you can list the three or four item being tested? and what makes the answer sufficient?

Time?
Rate of Speed?
Distance?
Any other?

Thanks.

Not sure I understand what you mean...

Anyway, we need to get \(t_x\).

From (1) we have that \(t_y=t_x+1\) and from (2) we have that \(t_y=60\), thus \(t_x=60-1=59\).

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.

The concept being tested here is RTD right?

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).

Yes, that's correct.

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.

We are given that Car X follows Car Y across a bridge that is ½ mile long. We need to determine the time, in seconds, it took Car X to travel the ½ mile across the bridge.

Statement One Alone:

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.

From statement one, we know that Car X gained 1 second on Car Y while the two cars traveled over the bridge. Thus, Car X took 1 second less than Car Y took to cross the bridge. However, without knowing the actual time it took Car Y to cross the bridge, we still don't know the number of seconds it took Car X to cross it. Statement one alone is insufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

Car Y traveled across the bridge at a constant speed of 30 miles per hour.

From statement two we can determine the time it took Car Y to cross the bridge.

time = distance/rate

time = (1/2)/30

time = 1/60 hour

Since 1 hour = 3,600 seconds, 1/60 hour = 3,600 x 1/60 = 60 seconds.

We know that Car Y took 60 seconds to travel across the bridge. However, we do not know anything about Car X, so statement two is insufficient to answer the question. We can eliminate answer choice B.

Statements One and Two Together:

Because we know that that Car Y took 60 seconds to travel across the bridge and Car X took 1 second less, we know that it took Car X 59 seconds to travel across the bridge.

The answer is C.

i am stuck at trying to infer that the difference between X's time and Y's time is 1 second from the first sentence..can't quite imagine what happened there..
Y starts travelling across the bridge .
3sec
X starts walking the bridge
Y finnishes
2 sec
X finnishes
....and ?

Car Y completes 1/2 mile in 1 min as it covers 30 miles in 60 min
Y takes 60 s
Now car X starts and takes 60-3+2 sec to remain in bridge i.e. 59 sec
Both these info are present in two statement given.none alone is sufficient.

Give kudos this helps.

Posted from my mobile device

Thanks ..maybe something obvious ..but still not clear to me ..
From both sentences :
Y 's speed 30m per h
But how to infer from the first sentence that X'time is less than Y's by 1 sec
. The only thing i'm sure of that X'speed is more than Y's.. because if say it was the same then the difference would still be 3 sec after Y ..so it is clearly faster. But by how much ?
How did you infer that time of X is 60(Y'time) -3+2 ?

Sent from my PRA-LA1 using GMAT Club Forum mobile app

Dear Bunuel

How did you infer or conclude that relation in statement 1? I'm not able to understand it.

Thanks

Highlighted portion should help. Car X was on the bridge 3 seconds after car Y, but off the bridge only 2 seconds after car Y. So, X was on the bridge 1 second less than car Y.