CREATE TABLE table_16043 ("Year" FLOAT, "Broadcast date" TEXT, "BBC One total viewing" FLOAT, "BBC One Rank" TEXT, "BBC Two total viewing" TEXT, "BBC Two Rank" TEXT)

How many years did BBC One rank 20th?
SELECT COUNT("Year") FROM table_16043 WHERE "BBC One Rank" = '20th'
SELECT COUNT("Year") FROM "table_16043" WHERE "BBC One Rank" = '20th'