why 70 :
Total number of wrong answers from all people is 90. For number of people having 3 or more correct answers to be minimal, the number of people making 3 or more mistakes should be maximal. So, these 90 mistakes should be made by 30 people, each of them makes 3 mistakes.
Total number of wrong answers from all people is 90. For number of people having 3 or more correct answers to be minimal, the number of people making 3 or more mistakes should be maximal. So, these 90 mistakes should be made by 30 people, each of them makes 3 mistakes.
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