Scenario :
In a Mathematics test, each student must answer 20 true or false questions. A student that is sitting for the test knows the correct answer for 8 questions but he tosses a coin to determine the answers for the remaining questions.
Question :
Find the probability that he can answer at least 90% of the questions correctly.
Answer provided in the book :
My way to answer :
(Since 8 questions are 100% correct, so I just do not count them in)
Which method is correct?
If the method 1 is correct, we have already known 8 questions are 100% correct, how can p be 0.7?
Shouldn't we exclude those 8 questions when counting the probability?