$32)$ Rockford Company manufactures a part for use in its production of hats. When $10,000$ items are produced, the costs per unit are:

Direct materials $$0\mathrm{.}75$

Direct manufacturing labor $3\mathrm{.}00$

Variable manufacturing overhead $1\mathrm{.}50$

Fixed manufacturing overhead $1\mathrm{.}60$

Total $$6\mathrm{.}85$

Angel Company has offered to sell to Rockford Company $10,000$ units of the part for $$6\mathrm{.}00$ per unit. The plant facilities could be used to manufacture another item at a savings of $$9,000$ if Rockford accepts the offer. In addition, $$1\mathrm{.}00$ per unit of fixed manufacturing overhead on the original item would be eliminated.

Required:

a$.$ What is the relevant per unit cost for the original part?

b$.$ Which alternative is best for Rockford Company? By how much?