how many questions should i study?

Question

for my upcoming religion exam, my teacher gave us 10 short answer questions to practice. 8 of those questions will be on the exam (we don’t know which ones). we only have to answer 5 of the 8.

what is the minimum amount of questions i can practice to be 100% prepared for the 5 questions?

Answer 1

Let me rephrase this problem a different way

In order to be prepared to answer 5 questions out of 8 (out of 10 total), you need to make sure that there are only 3 questions that you don't know the answer to

That way, worst case scenario, you get those 3 questions on the test then there are 5 other questions that you should have an answer for

So you want to prepare answers for all but 3 questions

Answer 2

Why not all of them?