Question: What was the percentage of the Bitcoins verifiably burned until 2018/4/28?
SQL: SELECT CAST((SUM(T2.max_supply) - SUM(T2.total_supply)) AS REAL) / SUM(T2.total_supply) FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE T2.date < '2018-04-28' AND T1.name = 'Bitcoin'