Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_16043 ( "Year" real, "Broadcast date" text, "BBC One total viewing" real, "BBC One Rank" text, "BBC Two total viewing" text, "BBC Two Rank" text )

### Question ###
How many years did BBC One rank 20th?

### Accurate SQL ###

SELECT COUNT("Year") FROM table_16043 WHERE "BBC One Rank" = '20th'