The problem seems to be very simple.
Perhaps I'm misinterpreting it?
There is an exam. There are 25 different exam papers
for 25 students who are lined up outside of the classroom.
Each student enters the room, randomly picks a paper and starts his exam.
You are the 5th person in the line, who has to enter the room and pick a paper.
Among those 25 papers, there is only ine paper whose questions are familiar to you.
What is the probability that you will enter the classroom and pick that one paper?
That is, the probability that NO other student before you will pick that exam paper.
Let that special exam paper be called the "Exam".
For you to get the Exam, it must be the fifth one selected.
The probability that the Exam is the fifth one chosen is: .
We can check this answer.
For you to get the Exam, the first four students must NOT get the Exam,
. . then you must get the Exam.
This probability is: .