user: Question: When was the person who became queen on 15 April 1820 born?
Columns: ['Name', 'Birth', 'Marriage', 'Became Queen', 'Ceased to be Queen', 'Death', 'Spouse']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Charlotte, Princess Royal', '29 September 1766', '18 May 1797', '1 January 1806 elevated from Electress', "30 October 1816 husband's death", '5 October 1828', 'Frederick I']
SQL:

assistant: SELECT "Birth" FROM "Table" WHERE "Became Queen" = '15 April 1820';