What was the percentage of the Bitcoins verifiably burned until 2018/4/28?
the percentage of the Bitcoins verifiably burned = divide(subtract(SUM(max_supply), SUM(total_supply)),SUM(total_supply))*100%; until 2013/4/28 refers to date<'2013-04-08'
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'