Explain that this 20% ticket is invalid.
Only when someone casts more than three votes at a time, the votes are invalid, and each person can cast at most five votes. In order to make voting most efficient, (note: efficient voting depends on the number of people who cast valid votes. After all, this person made a mistake, and all his votes were invalid), so we need to assume that the person who voted wrongly is the least, that is, the least person made the most mistakes, so the reason for all invalid votes is that this person actually voted five votes.
But this 20% is invalid, because it exceeds the total amount, that is to say, only those who voted wrongly account for this 20%, that is to say, the excess of 2 votes accounts for 20%. Counting the first three votes, in fact, these people's invalid votes accounted for 50% of the total votes. How many of these people who made mistakes voted for all five songs? 10%。 10% invalid people voted 5 votes each, so they voted 50%. The final effective person is 1- 10%=90%.