I wanted to know how the math for less: beginning inventory is solved.

Production Budget
The company maintains a finished goods inventory equal to 10% of the following
month's sales. The inventory of finished goods on July 1 is as it should be.
Vaughan Company
Production Budget				Total
3rd Quarter	July	August	September	3rd Quarter
Budgeted Sales in Units	4,000	20,000	12,000	36,000
Add: Desired Ending Inventory	2,000	1,200	400	400
Total Needs	6,000	21,200	12,400	36,400
Less: Beginning Inventory	(400)	(2,000)	(1,200)	(400)
Required Production	5,600	19,200	11,200	36,000