[Math] Train arrival Paradox

probabilityproof-explanationword problem

I am unable to understand the solution to this question: "At your subway station, you notice that of the two trains running in opposite directions which are supposed to arrive with the same frequency, the train going in one direction comes first 80% of the time, while the other train going in the opposite direction comes first 20% of the time. What do you think could be happening?

The solution provided is: " Assume that your arrival in the station is uniformly distributed. If both trains run every ten minutes and train A comes into the station at 1, 1:10, 1:20,..while train B comes at 1:12, 1:22, ..then train A will come first 80% of the time." I don't understand the logic here.

Best Answer

If the ones digit of the minutes of your arrival is 0 or 1, then you see the B train first. If the ones digit of the minutes of your arrival is 2, 3, 4, 5, 6, 7, 8, 9, then you see the A train first.

Your arrival time is uniformly distributed...

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