During August, Wilson Lubricant completed 31,000 gallons of product. At the start of August there were 4,500

Question:

During August, Wilson Lubricant completed 31,000 gallons of product. At the start of August there were 4,500 gallons in Work in Process, and at the end of August, there were 7,000 gallons in Work in Process.
Required
How many gallons of product were started during the month of August?