Among the matches in 2013, what is the percentage of winning of the team "Mumbai Indians"?
in 2013 refers to Match_Date like '2013%'; winning of the team "Mumbai Indians" refers to Match_Winner = 7; percentage refers to DIVIDE(COUNT(Match_Winner = 7), COUNT(Match_Winner))
SELECT CAST(SUM(CASE WHEN T2.Match_Winner = 7 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T2.Match_Winner) FROM Team AS T1 INNER JOIN Match AS T2 ON T1.Team_Id = T2.Match_Winner WHERE T2.Match_Date LIKE '2013%'