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Bunuel
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Bunuel could you please share more questions like this to pratice?

Check the questions in the link below:

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Bunuel
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Two cyclists start moving simultaneously from opposite ends of a straight track towards each other back and forth. Cyclists' speeds are constant, but one is faster than the other. First time, the cyclists meet each other at a distance of \(x\) meters from the nearest end of the track. Second time, on the way back, they meet \(y\) meters from the other end of the track. What is the length of the track?

Check the image below:



By the time of their first meeting, the total distance the two cyclists have covered equals to the the length of the track (check top image: A and B together covered 1 full length of the track).

By the time of their second meeting, the total distance the two cyclists have covered equals to three times the length of the track (check the lower image: A and B together covered 3 full lengths of the track).

Since the speeds are constant, the second meeting (after covering 3 full lengths of the track) occurs after a total time that is thrice the time for the first meeting (after covering 1 full length of the track). So, if by the time of their first meeting, cyclist A covered \(x\) meters, then by the time of their second meeting, cyclist A covered \(3x\) meters (3 times the distance in 3 times the time).

Now, check the lower image again, the total distance A covered (\(3x\) meters), is \(y\) meters more than the length of the track, so the length of the track is \(3x-y\) meters.

So, to get the length we only need to know the values of \(x\) and \(y\), while speeds are irrelevant.

(1) \(x = 720\) meters

Not sufficient.

(2) \(y = 400\) meters

Not sufficient.

(1)+(2) The length of the track is \(3x-y=3*720-400=1760\) meters. Sufficient.


Answer: C

Hi Bunuel , I am having some trouble in understanding how total distance travellled by A is 3x. I understood that till the second meet total distance covered is 3 times the total distance. But afterwards I got lost. Can you please break it down a little bit more
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rickyric395
Bunuel
Official Solution:


Two cyclists start moving simultaneously from opposite ends of a straight track towards each other back and forth. Cyclists' speeds are constant, but one is faster than the other. First time, the cyclists meet each other at a distance of \(x\) meters from the nearest end of the track. Second time, on the way back, they meet \(y\) meters from the other end of the track. What is the length of the track?

Check the image below:



By the time of their first meeting, the total distance the two cyclists have covered equals to the the length of the track (check top image: A and B together covered 1 full length of the track).

By the time of their second meeting, the total distance the two cyclists have covered equals to three times the length of the track (check the lower image: A and B together covered 3 full lengths of the track).

Since the speeds are constant, the second meeting (after covering 3 full lengths of the track) occurs after a total time that is thrice the time for the first meeting (after covering 1 full length of the track). So, if by the time of their first meeting, cyclist A covered \(x\) meters, then by the time of their second meeting, cyclist A covered \(3x\) meters (3 times the distance in 3 times the time).

Now, check the lower image again, the total distance A covered (\(3x\) meters), is \(y\) meters more than the length of the track, so the length of the track is \(3x-y\) meters.

So, to get the length we only need to know the values of \(x\) and \(y\), while speeds are irrelevant.

(1) \(x = 720\) meters

Not sufficient.

(2) \(y = 400\) meters

Not sufficient.

(1)+(2) The length of the track is \(3x-y=3*720-400=1760\) meters. Sufficient.


Answer: C

Hi Bunuel , I am having some trouble in understanding how total distance travellled by A is 3x. I understood that till the second meet total distance covered is 3 times the total distance. But afterwards I got lost. Can you please break it down a little bit more

At the first meeting point, cyclist A covered x meters, and together, both cyclists covered the length of 1 track.
At the second meeting point, cyclist A covered 3x meters, and together, they covered the length of 3 tracks.

This happens because by the second meeting point, three periods of time equivalent to the time to the first meeting have passed. Given that the speed of each cyclist is constant, the distance each covers is directly proportional to time. So, if cyclist A covers x meters for the first meeting, they cover 3x meters by the second meeting.

Hope it's clear.
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Certainly we need those 2 constants.

If we consider total distance as y+d+x (where d = distance between two meeting points) hence as speed is directly proportional to distance covered:

(y+d)/x= (2x+d)/(2y+d)

As we know values of x and y we can solve.
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Bunuel
Official Solution:


Two cyclists start moving simultaneously from opposite ends of a straight track towards each other back and forth. Cyclists' speeds are constant, but one is faster than the other. First time, the cyclists meet each other at a distance of \(x\) meters from the nearest end of the track. Second time, on the way back, they meet \(y\) meters from the other end of the track. What is the length of the track?

Check the image below:



By the time of their first meeting, the total distance the two cyclists have covered equals to the the length of the track (check top image: A and B together covered 1 full length of the track).

By the time of their second meeting, the total distance the two cyclists have covered equals to three times the length of the track (check the lower image: A and B together covered 3 full lengths of the track).

Since the speeds are constant, the second meeting (after covering 3 full lengths of the track) occurs after a total time that is thrice the time for the first meeting (after covering 1 full length of the track). So, if by the time of their first meeting, cyclist A covered \(x\) meters, then by the time of their second meeting, cyclist A covered \(3x\) meters (3 times the distance in 3 times the time).

Now, check the lower image again, the total distance A covered (\(3x\) meters), is \(y\) meters more than the length of the track, so the length of the track is \(3x-y\) meters.

So, to get the length we only need to know the values of \(x\) and \(y\), while speeds are irrelevant.

(1) \(x = 720\) meters

Not sufficient.

(2) \(y = 400\) meters

Not sufficient.

(1)+(2) The length of the track is \(3x-y=3*720-400=1760\) meters. Sufficient.


Answer: C
How do you know that x and y don't overlap? For example, if cyclist B is moving so slowly compared to A, that A catches up to B at a point that overlaps the x segment?
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ht1998
Bunuel
Official Solution:


Two cyclists start moving simultaneously from opposite ends of a straight track towards each other back and forth. Cyclists' speeds are constant, but one is faster than the other. First time, the cyclists meet each other at a distance of \(x\) meters from the nearest end of the track. Second time, on the way back, they meet \(y\) meters from the other end of the track. What is the length of the track?

Check the image below:



By the time of their first meeting, the total distance the two cyclists have covered equals to the the length of the track (check top image: A and B together covered 1 full length of the track).

By the time of their second meeting, the total distance the two cyclists have covered equals to three times the length of the track (check the lower image: A and B together covered 3 full lengths of the track).

Since the speeds are constant, the second meeting (after covering 3 full lengths of the track) occurs after a total time that is thrice the time for the first meeting (after covering 1 full length of the track). So, if by the time of their first meeting, cyclist A covered \(x\) meters, then by the time of their second meeting, cyclist A covered \(3x\) meters (3 times the distance in 3 times the time).

Now, check the lower image again, the total distance A covered (\(3x\) meters), is \(y\) meters more than the length of the track, so the length of the track is \(3x-y\) meters.

So, to get the length we only need to know the values of \(x\) and \(y\), while speeds are irrelevant.

(1) \(x = 720\) meters

Not sufficient.

(2) \(y = 400\) meters

Not sufficient.

(1)+(2) The length of the track is \(3x-y=3*720-400=1760\) meters. Sufficient.


Answer: C
How do you know that x and y don't overlap? For example, if cyclist B is moving so slowly compared to A, that A catches up to B at a point that overlaps the x segment?
There’s no overlap issue because x and y refer to positions along the same fixed-length track, not overlapping paths.

  • At first meeting: A covers x, B covers the rest, so total distance = full track.
  • At second meeting: they’ve together covered 3 full lengths. Since A is faster and has constant speed, A must have covered 3x by then.
  • In the diagram, A is shown meeting B y meters from the other end. That’s a fixed position on the track.

So, the logic is: A’s total distance (3x) = full track + extra beyond 1 round trip = track + (track - y)

Solving gives: 3x = 2*track - y, so track = 3x - y.

There’s no case where x and y “overlap” in the sense of causing ambiguity. They’re distinct fixed points on a linear track, each defined at a specific moment in the cycle.
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What about the scenario where B is so slow that A returns back and meets B before B even reaches the opposite end?

Why are we considering that B reached the other end and returned what if A met B twice before B could even travel once
Bunuel
ht1998
Bunuel
Official Solution:


Two cyclists start moving simultaneously from opposite ends of a straight track towards each other back and forth. Cyclists' speeds are constant, but one is faster than the other. First time, the cyclists meet each other at a distance of \(x\) meters from the nearest end of the track. Second time, on the way back, they meet \(y\) meters from the other end of the track. What is the length of the track?

Check the image below:



By the time of their first meeting, the total distance the two cyclists have covered equals to the the length of the track (check top image: A and B together covered 1 full length of the track).

By the time of their second meeting, the total distance the two cyclists have covered equals to three times the length of the track (check the lower image: A and B together covered 3 full lengths of the track).

Since the speeds are constant, the second meeting (after covering 3 full lengths of the track) occurs after a total time that is thrice the time for the first meeting (after covering 1 full length of the track). So, if by the time of their first meeting, cyclist A covered \(x\) meters, then by the time of their second meeting, cyclist A covered \(3x\) meters (3 times the distance in 3 times the time).

Now, check the lower image again, the total distance A covered (\(3x\) meters), is \(y\) meters more than the length of the track, so the length of the track is \(3x-y\) meters.

So, to get the length we only need to know the values of \(x\) and \(y\), while speeds are irrelevant.

(1) \(x = 720\) meters

Not sufficient.

(2) \(y = 400\) meters

Not sufficient.

(1)+(2) The length of the track is \(3x-y=3*720-400=1760\) meters. Sufficient.


Answer: C
How do you know that x and y don't overlap? For example, if cyclist B is moving so slowly compared to A, that A catches up to B at a point that overlaps the x segment?
There’s no overlap issue because x and y refer to positions along the same fixed-length track, not overlapping paths.

  • At first meeting: A covers x, B covers the rest, so total distance = full track.
  • At second meeting: they’ve together covered 3 full lengths. Since A is faster and has constant speed, A must have covered 3x by then.
  • In the diagram, A is shown meeting B y meters from the other end. That’s a fixed position on the track.

So, the logic is: A’s total distance (3x) = full track + extra beyond 1 round trip = track + (track - y)

Solving gives: 3x = 2*track - y, so track = 3x - y.

There’s no case where x and y “overlap” in the sense of causing ambiguity. They’re distinct fixed points on a linear track, each defined at a specific moment in the cycle.
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thisisakr
What about the scenario where B is so slow that A returns back and meets B before B even reaches the opposite end?

Why are we considering that B reached the other end and returned what if A met B twice before B could even travel once


First of all, check the image: the first meeting happens x meters from A’s side, so B is faster than A, not the other way around.

Second, the question clearly says “on the way back”, which means both cyclists reached the opposite end and turned around.
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I don’t quite agree with the solution. I used the same logic to get 3x-y but this could also be 3x+y (consider a case when B is way more faster than A -> In this case A never reaches the other end before they both meet). Hence I marked E
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Shrinathda
I don’t quite agree with the solution. I used the same logic to get 3x-y but this could also be 3x+y (consider a case when B is way more faster than A -> In this case A never reaches the other end before they both meet). Hence I marked E

Check this:

“Second time, on the way back, they meet y meters from the other end.”

That line means both cyclists have already turned and are returning. So the second meeting happens after each has touched the far wall once.
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Yeah, I was considering C as well, based on how to infer that line. For e.g. I can also infer it to be -> on the way back for one of them -> maybe I am overthinking this, but I get why C is right.
Bunuel


Check this:

“Second time, on the way back, they meet y meters from the other end.”

That line means both cyclists have already turned and are returning. So the second meeting happens after each has touched the far wall once.
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