Data Sufficiency

please help on this one


Stations X and Y are connected by two separate, straight, parallel rail lines that are 250 miles long. Train P and train Q simultaneously left station X and station Y, respectively, and each train traveled to the other's points of departure.The 2 trains passed each other after traveling for 2 hours, when the two trains passed, which train was nearer to its destination

There is some missing information. Probably the speed of trains.

This is called data insufficiency problem ( never mind )

papgust wrote:There is some missing information. Probably the speed of trains.

It's DS, so I assume that the statements are what's missing!

You're correct though - to solve, what we really need is the relative speed of the trains (we don't need the actual speeds, we just need to know which one is faster - that one will be closer to its destination when they meet).

This is the original question:

Stations X and Y are connected by two separate, straight, parallel rail lines that are 250
miles long. Train P and train Q simultaneously left Station X and Station Y, respectively,
and each train traveled to the other's point of departure. The two trains passed each other
after traveling for 2 hours. When the two trains passed, which train was nearer to its
destination?

(1) At the time when the two trains passed, train P had averaged a speed of 70 miles
per hour.

(2) Train Q averaged a speed of 55 miles per hour for the entire trip.

What do you think Stuart?

1) is not sufficient as relative velocity is needed to determined which travelled more.

2) Not sure what is the speed of other train.

1 & 2 can is enough to solve the problem.

If Q avg speed is 55 mph and both together is 70mph, P should be travelling faster and should be nearr to its destination.

Is the answer C?

dear_xavier wrote:This is the original question:

Stations X and Y are connected by two separate, straight, parallel rail lines that are 250
miles long. Train P and train Q simultaneously left Station X and Station Y, respectively,
and each train traveled to the other's point of departure. The two trains passed each other
after traveling for 2 hours. When the two trains passed, which train was nearer to its
destination?

(1) At the time when the two trains passed, train P had averaged a speed of 70 miles
per hour.

(2) Train Q averaged a speed of 55 miles per hour for the entire trip.

What do you think Stuart?

Let's start by analyzing the problem:

They started 250 miles apart, so they collectively traveled 250 miles by the time they passed.

d=250, t=2, therefore their combined rate is 125 mph for those 2 hours.

Next, let's evaluate the statements:

(1) Train P averaged 70mph for the first two hours.

Since Rate P + Rate Q = 125 for those two hours, we can now calculate Rate Q. If we know the 2 rates, we can definitely answer the question: sufficient.

(2) Knowing Q's average speed for the entire trip (i.e. all the way from X to Y) doesn't tell us Q's average speed for the first 2 hours of the trip: insufficient.

(1) is sufficient, (2) isn't: choose A.

I am slightly confused, the problem does not say that after traveling two hours they have reached their destinations hence the assumption of both trains traveling two hours does not imply the journey is finished. Hence the total time taken to travel 250 kms is not correct
Pls provide a clear answer to this assumption
regards
Sravan

Sorry
Was not keen in reading so was confused. Your answer is correct
Sravan

Stuart Kovinsky wrote:

dear_xavier wrote:This is the original question:

Stations X and Y are connected by two separate, straight, parallel rail lines that are 250
miles long. Train P and train Q simultaneously left Station X and Station Y, respectively,
and each train traveled to the other's point of departure. The two trains passed each other
after traveling for 2 hours. When the two trains passed, which train was nearer to its
destination?

(1) At the time when the two trains passed, train P had averaged a speed of 70 miles
per hour.

(2) Train Q averaged a speed of 55 miles per hour for the entire trip.

What do you think Stuart?

Let's start by analyzing the problem:

They started 250 miles apart, so they collectively traveled 250 miles by the time they passed.

d=250, t=2, therefore their combined rate is 125 mph for those 2 hours.

Next, let's evaluate the statements:

(1) Train P averaged 70mph for the first two hours.

Since Rate P + Rate Q = 125 for those two hours, we can now calculate Rate Q. If we know the 2 rates, we can definitely answer the question: sufficient.

(2) Knowing Q's average speed for the entire trip (i.e. all the way from X to Y) doesn't tell us Q's average speed for the first 2 hours of the trip: insufficient.

(1) is sufficient, (2) isn't: choose A.

should not we consider the lengths of the train since we are talking about the two trains passing each other?

ansumania wrote:
should not we consider the lengths of the train since we are talking about the two trains passing each other?

You never have to worry about this on the GMAT - assume that the trains have length 0 (or that you're only concerned with the exact time that the front of the trains meet).