Final answer:
The expression for a 15% discount on a pen that costs $c is 0.15c, which represents the value of the discount.
Step-by-step explanation:
The student has asked how to calculate the value of a discount given on a pen that originally costs $c and is now on sale with a 15% discount. To find the value of the discount, you multiply the original price by the discount rate. The expression for the value of the discount can be represented as: 0.15c.
The discount rate is 15%, which is 0.15 when converted into decimal form. Therefore, if a pen costs $c, the value of the discount given on the pen is 0.15 multiplied by $c (0.15c). This expression shows the amount of money that will be subtracted from the original price to obtain the sale price.