CREATE TABLE "second_round_proper" (
  "tie_no" text,
  "home_team" text,
  "score" text,
  "away_team" text,
  "attendance" real
);

Question: What was the attendance for the game that has a tie number of 13?
SQL: SELECT SUM("attendance") FROM "second_round_proper" WHERE "tie_no"='13';

Question: What is the average attendance when the Forest Green Rovers is the away team?
SQL: SELECT AVG("attendance") FROM "second_round_proper" WHERE "away_team"='forest green rovers';