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They could all be correct because it depends on the peron's position in the train.

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They could all be correct, it just depends on the frame of reference for each.

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It is possible that they are all correct based on their viewpoints. Since we do not where they are located we cannot immediately determine if their viewpoint is correct or incorrect.

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I think Alex and Mike might be correct since their frame of reference are different.

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All three people could be correct. There is not enough information given to determine where the three people are in relation to the train.

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Frame of reference is key here. They all 3 can be correct it is just dependent on how they are observing the train. Jenny could be in the train. Alex could be watching the train. And mike is moving parallel to the train traveling 60 m/s

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No details are given to necessarily prove one wrong. Frame of reference is the main issue here as the 3 characters could each be moving at 120mph (inside the train), 60mph, or 0mph resulting in the difference between their answers.

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They could all be correct depending on where they are in relation to the train. They could all also be wrong at the same time.

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They all might be correct because they are all at different positions looking at the train. If you are looking at the train, and not standing still you will get a different miles per hour than a person that is standing still looking  at the train. Jenny is going at the same speed of the train or inside it, Alex is just standing still looking at the train, and Mike might be looking at the train in another train coming the opposite directions, that train Mike is on is going slower.

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Any might be correct relative to where they are either on the train or outside of the train. It all depends on the frame of reference they are using and you can't determine the correct answer until you know the frame of reference.

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Any of them could be correct. The answer depends on their frame of references. One explanation for the differing answers is that Jenny is moving away from Alex at 120 m/s and that Mike is moving away from Alex at 60 m/s.

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All three people are correct from their own viewpoints.  Alex is the "most" correct from a complete outsiders' viewpoint.

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They could all be correct depending on the frame of reference. The train would be going 0 mph for a passenger on the train, it would be going 120mph for someone standing on the side of the tracks, and it would be going 60 mph for a passenger on another train going 60mph in the same direction (according to someone on the ground).

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Each rider could be proven correct or incorrect depending on where they are situated in regards to the train, or their frame of reference. For example, someone on the train would report that they are feeling close to no speed at all, while a person watching the train move by would report its actual speed as the speed they are experiencing.

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-Alex is observing the train from a distance
-Jenny is in the train
-Mike is in an object/car that is nearby the train and he is driving at 60mph, thus he can only observe the Shinkansen to be going at 60mph