CREATE TABLE table_6116 ("Name" TEXT, "Birth" TEXT, "Marriage" TEXT, "Became Queen" TEXT, "Ceased to be Queen" TEXT, "Death" TEXT, "Spouse" TEXT)

When was the person who became queen on 15 April 1820 born?
SELECT "Birth" FROM table_6116 WHERE "Became Queen" = '15 april 1820'
SELECT "Birth" FROM "table_6116" WHERE "Became Queen" = '15 april 1820'