In an exam with 12 yes/no questions with 8 correct needed to pass, is it better to answer randomly or answer exactly 6 times yes and 6 times no, given that the answer ‘yes’ is correct for exactly 6 questions?

I have calculated the probability of passing by guessing randomly and it is

12∑k=8(12k)0.5k0.5n−k=0.194

Now given that the answer ‘yes’ is right exactly 6 times, is it better to guess ‘yes’ and ‘no’ 6 times each?

My idea is that it can be modelled by drawing balls without replacement. The balls we draw are the correct answers to the questions.

Looking at the first question, we still know that there are 6 yes and no’s that are correct. The chance that a yes is right is 612 and the chance that a no is right is also 612.

Of course the probability in the next question depends on what the first right answer was. If yes was right, yes will be right with a probability of 5/11 and a no is right with the chance 6/11. If no was right, the probabilities would change places.

Now that we have to make the choice 12 times and make the distinction which one was right, we get 212 paths total. We cannot know what the correct answers to the previous questions were. So we are drawing 12 balls at once, but from what urn? It cannot contain 24 balls with 12 yes and 12 no’s. Is this model even correct?

Is there a more elegant way to approach that?

I am asking for hints, not solutions, as I’m feeling stuck. Thank you.

Edit: After giving @David K’s answer more thought, I noticed that the question can be described by the hypergeometric distribution, which yields the desired result.

**Answer**

We are given the fact that there are 12 questions, that 6 have the correct answer “yes” and 6 have the correct answer “no.”

There are (126)=924 different sequences of 6 “yes” answers and 6 “no” answers.

If we know nothing that will give us a better chance of answering any question

correctly than sheer luck, the most reasonable assumption is that every possible sequence of answers is equally likely, that is, each one has

1924 chance to occur.

So guess “yes” 6 times and “no” 6 times. I do not care how you do that:

you may guess “yes” for the first 6, or flip a coin and answer “yes” for heads and “no” for tails until you have used up either the 6 “yeses” or the 6 “noes” and the rest of your answers are forced, or you can put 6 balls labeled “yes” and 6 labeled “no” in an urn, draw them one at a time, and answer the questions in that sequence.

No matter *what* you do, you end up with some sequence of “yes” 6 times and “no” 6 times. You get 12 correct if and only if the sequence of correct answers is exactly the same as your sequence.

That probability is 1924.

There is no way for you to get 11 correct. You get 10 correct if and only if the correct answers are “yes” on 5 of your “yes” answers and “no” on your other “yes” answers.

The number of ways this can happen is the number of ways to choose 5 correct answers from your 6 “yes” answers, times the number of ways to choose 5 correct answers from your 6 “no” answers:

(65)×(65)=36.

There is no way for you to get 9 correct. You get 8 correct if and only if the correct answers are “yes” on 4 of your “yes” answers and “no” on your other “yes” answers.

The number of ways this can happen is the number of ways to choose 4 correct answers from your 6 “yes” answers, times the number of ways to choose 4 correct answers from your 6 “no” answers:

(64)×(64)=225.

In any other case you fail. So the chance to pass is

1+36+225924=131462≈0.283550,

which is much better than the chance of passing if you simply toss a coin for each individual question

but not nearly as good as getting 4 or more heads in 6 coin tosses.

Just to check, we can compute the chance of failing in the same way:

6 answers correct (3 “yes” and 3 “no”), 4 answers correct,

2 correct, 0 correct. This probability comes to

(63)2+(62)2+(61)2+1924=400+225+36+1924=331462≈0.716450,

which is the value needed to confirm the answer above.

**Attribution***Source : Link , Question Author : B.Swan , Answer Author : David K*