user: Question: Name the total number of previous br number for number 21
Columns: ['Number', 'Previous BR No.', 'Taken into deptal stock', 'Withdrawn', 'Disposal']
Types: ['text', 'real', 'real', 'real', 'text']
Sample row: ['17', 61059.0, 1963.0, 1966.0, 'Scrapped (1966)']
SQL:

assistant: SELECT SUM("Previous BR No.") FROM "Table" WHERE "Number" = '21';