I have the right solutions for thoses questions but i need to know how to calculate it! The bold answer is the correct one.

A chocolate manufacturer covers its annual demand for packaging units (45,000 units) by placing an order at the beginning of the year. The units cost CHF 25 each. Each order incurs an expense of CHF 1,000. The inventory rate is 20% p.a., and a safety stock is not taken into account.

a) By how much will the total annual procurement costs be reduced if the current order quantity (45,000 units) is changed to the optimal order quantity? A. CHF 7,456 B. CHF 25,217 C. CHF 51,753 D. CHF 92,287 E. CHF 125,121

b) The cost of an order can be reduced to CHF 250. How does the optimal order quantity change? A. The order quantity triples. B. The order quantity is halved. C. The order quantity drops to a quarter of the previous value. D. The order volume increases by a quarter. E. The order quantity doubles.

c) An additional 7,500 rolls of special foil are required for production. The units cost CHF 250 per piece. The ordering cost and stock rate remain unchanged at CHF 1,000 and 20% per year, respectively. How many rolls are ordered at optimal cost per order? A. 548 pieces B. 647 pieces C. 812 pieces D. 1,023 pieces E. 1,211 pieces