Assume I want to pass a test with 98 % probability, or 95 % probability or 90 % probability or even as low as with 75 % probability.
We say that I will take the test if the probability of success is at least 75 %.
The test has 80 questions. (It is the OCAJP 8 test.) I have a practice test with 77 questions. Now if I'm lazy I don't answer all 77 questions on the practice (mock) test but only as many as I need to estimate my probability of success.
For instance, I answer 4 mock questions and get 3 answers right, then it is too little sample size to assume that I will pass the test (where passing score is 65 %) and too little sample size to generalize to 75 %.
I know there is a "magic sample size" of about 30 questions that seems reasonable here. If I answer 30 questions and get 28 or 29 answers right then it will seem "reasonable" that I will get at least 65 % on the real test.
But how should I make a more exact and correct estimate? How many questions do I have to answer to generalize my mock score?