user: Question: What is the average number for northgate which commenced its construction before 1951?
Columns: ['Number on map', 'Name', 'Colour', 'Construction commenced', 'Population']
Types: ['real', 'text', 'text', 'real', 'real']
Sample row: [1.0, 'Langley Green', 'Grey', 1952.0, 7286.0]
SQL:

assistant: SELECT AVG("Number on map") FROM "Table" WHERE "Name" = 'northgate' AND "Construction commenced" < 1951.0;