Which Date has a 1946 Nos of 3156/7?
CREATE TABLE "construction" (
  "1946_nos" text,
  "1947_nos" text,
  "date" real,
  "built_at" text,
  "lms_nos" text,
  "br_nos" text
);
SELECT AVG("date") FROM "construction" WHERE "1946_nos"='3156/7';