Table: forbes_global_2000_6
Columns: rank
company
headquarters
industry
sales_billion
profits_billion
assets_billion
market_value_billion

Question: What is the average sales in billions of walmart, which has more than 15.7 billion in profits?
SQL: SELECT AVG("sales_billion") FROM forbes_global_2000_6 WHERE "company" = 'walmart' AND "profits_billion" > 15.7