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I am looking at a question from STEP and I am a little confused by the logic of the method, and I really hope that someone can clarify what is happening to me. I have a good knowledge (at least I thought so), as some questions from STEP II and III are accessible, but this one, I just can not understand, there must be a gap in my understanding .

It would be difficult for me to rewrite the question here but here is the video of the question AND the solution (it's not too long, I assure you, the first two minutes is the entry of the question which I scratch my head)

I understand the calculation later in the question about waiting and even the very last part of the question. I just have a hard time understanding how it makes sense to consider what this guy has considered in the first two minutes of the video. . Is there a nicer explanation for which he considered P (T> t) and not P (T

I see what he has done and I understand it, it is simply something that I would have never even considered and that makes me feel uncomfortable.

Thank you for your help, and I apologize for the awkwardness of the question.