elementary math

Val and I were at Half Priced Books today and overheard this
conversion between an employee and a mid 30's, overweight, male
customer.

Cust: How much are the Dungeons and Dragons books?
HPB: They're individually priced, but they're all 20% off.
Cust: It's $9.98 so how much is the discount?
HPB: $2 (presumably with a look of incredulity)