can someone explain how i do this 25% thing

Expected sales are planned as follows: June July August Sept October 38000 units 42000 units 51000 units 40000 units 37000 units Management wants each month's ending finished goods to equal 25% of next months sales a) Prepare a production budget for July - September b) if each item cost $ 15 to produce, what is the cost of production from July-September