# How many exam questions do I have to answer ? Process of elimination [closed]

I have 25 questions to study 8 of these will be on the exam I only have to write about 2

How many exam questions can I " eliminate " from studying

• It's not clear what you mean here. In what scenario could you "eliminate" even one question from studying? Dec 11, 2016 at 22:51
• not all 25 of the questions will be on the exam . Only 8/25 will . And of those I get to PICK 2 . So if I prepare the questions in advance . I won't need to study some of them . Dec 11, 2016 at 22:54
• Yes! Exactly !!! That's the answer I need Dec 11, 2016 at 22:58
• @Peter I think the fact that you get to choose 2 of the 8 is significant Dec 11, 2016 at 22:58
• Knowing 19 out of the 25 guarantees getting 2 right, if 8 questions are presented and the rest may be ignored Dec 11, 2016 at 23:00

If you know $k \leq 19$ out of the $25$ questions, then the probability of getting at least two questions you know is given by $$1 - \frac{18\cdot\binom{25-k}{7}}{\binom {25}{8}}$$ If you know $11$, there is approximately a $94\%$ chance of success. Around $90\%$ if you know $10$. See WA.