Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_42 (number_on_map INTEGER, name VARCHAR, construction_commenced VARCHAR)
### Question:
What is the average number for Northgate which commenced its construction before 1951?
### Answer:
SELECT AVG(number_on_map) FROM table_name_42 WHERE name = "northgate" AND construction_commenced < 1951