Promotional Novelties manufactures a wide range of novelty items for its corporate customers, It has just received

Question:

Promotional Novelties manufactures a wide range of novelty items for its corporate customers, It has just received an order for 20,000 toy tractor-trailers that will be sold by a gas station chain as part of a holiday promotion. The order is to be shipped at the beginning of week 9. The product structure tree below shows various components of the tractor-trailers and the way it is assembled.

The company can complete final assembly of the tractor-trailers at the rate of 10,000 a week. Assembly' time of Tractors and Trailers for this size order, in different departments, is one week each.
The tractors and trailers each have their own chassis departments that can produce chassis at the rate of 100,000 a week. Wheels are purchased from a supplier. Ordered wheels come in sets of 6,400. The lead time for delivery of wheels from the supplier is expected to be three weeks. Perform MRP for the wheels and its parents and answer the following questions:
a. How many wheels will the manager need to order?
b. When should the manager place the order?
c. How many wheels should the manager expect to have left over?