Providing that all the other questions are correct then it is 9 questions
.8 x 30 = 24 so you can miss 6 questions
40
24 of them.
35
6 of them.
The answer will depend on how many of the ones you do manage to answer are correct!
25 questions missed
18
20
No more than 15.But why miss any ?
You can miss 10% of questions, or 12 questions
To get exactly 90% you need to miss 5.6 questions.