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Hi Bunuel,

I feel the correct answer choice should be B , because from the second option we know that the train from Station A to B will be on track for 2 hrs i.e from 11:05 to 1:05 , where in the question it mentioned that one train per departure starting from 8, which means another train would depart at 8:15 , next 8:30 , 8:45 and then at 9 . So in one hr we have 5 train similarly from 11:05 to 12 we would have 4 trains , and from 12:15 to 1:05 we would have 4 trains . Hence 8 would be the total number of trains travelling from A to B the train from B to A will encounter.

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Trains depart from Station A to Station B every 15 minutes (one train per departure), starting from 8:00 am, and travel at the same constant speed.. At 11:05 am, a train, traveling at a constant speed, departs from Station B to Station A. If the distance between the two stations is 80 kilometers, how many trains traveling from A to B will the train traveling from B to A encounter during its journey?

(1) Trains from Station A to Station B travel at a constant speed of 40 kilometers per hour.

The above implies that trains from Station A to Station B are on the track for 2 hours. Therefore, the train from Station B is guaranteed to meet the 8 trains from Station A that departed between 9:15 a.m. and 11:00 a.m. If the train from B to A can cover the 80-kilometer distance in just 1 minute, it will only meet these 8 trains since it will arrive at Station A before the next train at 11:15 a.m. departs from Station A. However, if the train from B to A is much slower, it will meet more than 8 trains from Station A. Therefore, statement (1) is not sufficient to answer the question.

(2) The train from Station B to Station A travels at a constant speed of 40 kilometers per hour.

Since the train from Station B to Station A is on the track for 2 hours, it will arrive at Station A at 1:05 p.m. If trains from Station A to Station B take more than 3 hours and 5 minutes to arrive at Station B, then the train from B to A will meet all 21 trains that depart from Station A to Station B between 8:00 a.m. and 1:05 p.m. However, if the train from A to B can cover 80 kilometers in 1 minute, then the train from B to A will meet only 8 trains that depart from Station A to Station B between 11:15 a.m. and 1:05 p.m. Therefore, statement (2) is not sufficient to answer the question.

(1)+(2) We have no unknowns left, and can solve the question. However, for those who are interested, we can proceed as follows. Both, trains from Station A to B and the train from Station B to A are on the track for 2 hours. Therefore, the train traveling from B to A, in 2 hours, between 11:05 a.m. and 1:05 p.m., will encounter all 16 trains from Station A that departed between 9:15 a.m. and 1:05 p.m. Sufficient.

Answer: C
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Hi Bunuel,

I feel the correct answer choice should be B , because from the second option we know that the train from Station A to B will be on track for 2 hrs i.e from 11:05 to 1:05 , where in the question it mentioned that one train per departure starting from 8, which means another train would depart at 8:15 , next 8:30 , 8:45 and then at 9 . So in one hr we have 5 train similarly from 11:05 to 12 we would have 4 trains , and from 12:15 to 1:05 we would have 4 trains . Hence 8 would be the total number of trains travelling from A to B the train from B to A will encounter.

Bunuel
Official Solution:


Trains depart from Station A to Station B every 15 minutes (one train per departure), starting from 8:00 am, and travel at the same constant speed.. At 11:05 am, a train, traveling at a constant speed, departs from Station B to Station A. If the distance between the two stations is 80 kilometers, how many trains traveling from A to B will the train traveling from B to A encounter during its journey?

(1) Trains from Station A to Station B travel at a constant speed of 40 kilometers per hour.

The above implies that trains from Station A to Station B are on the track for 2 hours. Therefore, the train from Station B is guaranteed to meet the 8 trains from Station A that departed between 9:15 a.m. and 11:00 a.m. If the train from B to A can cover the 80-kilometer distance in just 1 minute, it will only meet these 8 trains since it will arrive at Station A before the next train at 11:15 a.m. departs from Station A. However, if the train from B to A is much slower, it will meet more than 8 trains from Station A. Therefore, statement (1) is not sufficient to answer the question.

(2) The train from Station B to Station A travels at a constant speed of 40 kilometers per hour.

Since the train from Station B to Station A is on the track for 2 hours, it will arrive at Station A at 1:05 p.m. If trains from Station A to Station B take more than 3 hours and 5 minutes to arrive at Station B, then the train from B to A will meet all 21 trains that depart from Station A to Station B between 8:00 a.m. and 1:05 p.m. However, if the train from A to B can cover 80 kilometers in 1 minute, then the train from B to A will meet only 8 trains that depart from Station A to Station B between 11:15 a.m. and 1:05 p.m. Therefore, statement (2) is not sufficient to answer the question.

(1)+(2) We have no unknowns left, and can solve the question. However, for those who are interested, we can proceed as follows. Both, trains from Station A to B and the train from Station B to A are on the track for 2 hours. Therefore, the train traveling from B to A, in 2 hours, between 11:05 a.m. and 1:05 p.m., will encounter all 16 trains from Station A that departed between 9:15 a.m. and 1:05 p.m. Sufficient.

Answer: C
Your reasoning overlooks a key point: the trains from Station A to B that the train from B to A will meet are not only the ones that depart after 11:05 a.m., it also meets trains that departed before 11:05 a.m. but are still on the track.

That’s why Statement (2) alone is not sufficient. It only tells us how long the train from B to A is on the track, but we don’t know how long trains from A to B are on the track, so we can’t figure out which ones are still traveling when the train from B to A starts.

Only when both statements are combined do we know that both directions take 2 hours, and can determine exactly how many trains are on the track during the B-to-A train's journey.

So the correct answer is C, not B.
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Bunuel, could you please explain the calculation for number of train occurence
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Bunuel, could you please explain the calculation for number of train occurence

Sure. Here's the calculation for how many trains the B-to-A train will encounter once both statements are used:

We are told:

  • Trains leave Station A every 15 minutes starting from 8:00 am.
  • The train from Station B to Station A departs at 11:05 am.
  • From both statements, we know that the travel time in either direction is 2 hours.

So, the B-to-A train is on the track from 11:05 am to 1:05 pm.

To find how many A-to-B trains it will meet, we need to count all trains that are on the track at any time during that window. That includes:

  1. Trains from Station A that departed between 9:15 am and 11:00 am – because they’re still on the track when the B-to-A train departs at 11:05 am (they each stay on the track for 2 hours).
  2. Trains that depart between 11:15 am and 1:05 pm – these start after the B-to-A train departs but will be on the track while it's still traveling.

Trains from A to B are spaced every 15 minutes:

  • From 9:15 am to 11:00 am: that's 8 trains
  • From 11:15 am to 1:05 pm: that's another 8 trains

So:

8 (before) + 8 (after) = 16 trains met in total.
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