What is the average sales in billions of walmart, which has more than 15.7 billion in profits?
CREATE TABLE "2012_list" (
  "rank" real,
  "company" text,
  "headquarters" text,
  "industry" text,
  "sales_billion" real,
  "profits_billion" real,
  "assets_billion" real,
  "market_value_billion" real
);
SELECT AVG("sales_billion") FROM "2012_list" WHERE "company"='walmart' AND "profits_billion">15.7;