Purchase$-$related transactions

The Stationery Company purchased merchandise on account from a supplier for $$9,000,$ terms $\frac{1}{10},\frac{n}{30}.$ The Stationery Company returned merchandis with an invoice amount of $$1,100$ and received full credit.

a$.$ If The Stationery 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 credited by The Stationery Company to record the return?