CREATE TABLE table_6864 ("Number on map" FLOAT, "Name" CLOB, "Colour" CLOB, "Construction commenced" FLOAT, "Population" FLOAT)

What is the average number for Northgate which commenced its construction before 1951?
SELECT AVG("Number on map") FROM table_6864 WHERE "Name" = 'northgate' AND "Construction commenced" < '1951'
SELECT AVG("Number on map") FROM "table_6864" WHERE "Construction commenced" < '1951' AND "Name" = 'northgate'