Expected sales are:
Units
July 14700
August 15120
September 15680
October 16240
Each month's closing inventory is to be 150% of the following month's cost of sales. Average selling price is $30 per unit. Average cost of purchases as $20 per unit.
How to calculate cost of sales of each month?

Cost of goods sold = sales - gross profit
14700 * (30-20) = 147000
Now based on sales you can work out purchases for each month. Foer example for June
Sales 14700 units
+Ending inventory 15120 * 1.5
- Beginning inventory 14700 * 1.5
=16125 units

Now you can work out COGS budget for each month for each month/