$23.$ On June $1,2011,$ merchandise subject to terms $2/10,$ n$/30$ was sold on account to a customer for $$23,500.$ On June $3,$ the seller issued a credit memorandum for $$5,100,$ accepting merchandise returned by the customer. This was prior to payment by the customer.$$

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a$)$ What is the amount of cash collected by the seller if the payment is made by the customer on June $8,2011?$

b$)$ What is the amount of cash collected by the seller if payment is made by the customer on June $21,2011?$