Kosmos Company purchased merchandise on account from a supplier for $21,500, terms 1/10, n/30.Kosmos Company returned $1,500

Question:

Kosmos Company purchased merchandise on account from a supplier for $21,500, terms 1/10, n/30.Kosmos Company returned $1,500 of the merchandise and received full credit.
a. If Kosmos Company pays the invoice within the discount period, what is the amount of cash required for the payment?
b. Under a perpetual inventory system, what account is debited by Kosmos Company to record the return?