The solution to the whole problem is this: ask at least how many people have passed the exam. If the positive thinking is complicated, think backwards-that is, ask first: how many people can't pass the exam at most.

Then let's see, what is the condition of not passing, that is, more than three questions are wrong. If we want to make as many people make mistakes as possible, then everyone should make as few mistakes as possible (but not less than 3 questions), so the extreme situation is that everyone who has not made mistakes will make 3 questions. Because there are always 90 wrong questions, 90÷3=30, so at most 30 people fail.

At most 30 people failed, which in turn means that at least 70 people must have passed.

There is another situation of this problem. For example, if the number of errors in 1 ~ 5 is changed to 20,8, 14,17,31,the answer is the same. Because although the same * * * wrong 90 questions, the result of using 90÷3 is also 30, but it should be noted that it is impossible to allocate 30 people to the wrong 3 1 question of the fifth question, which means that there must be 3 1 person wrong in the fifth question, then one of the top 30 people must have made only two mistakes, that is, only 29 people failed. The answer to the original question is "at least 7 1 person passed the exam"

Analyze the problem comprehensively.