In a knock-out tournament, each match results in the elimination of one team, except for the final match that determines the champion. For 21 teams, the number of matches required to determine the winner can be calculated as follows:
To find the total number of matches in a knockout tournament, we consider:
Thus, the total number of matches played, including the third-place match, is:
Total matches = 20 (to find the winner) + 1 (third-place match) = 21 matches
Therefore, the correct answer is 21.