Discount Calculation for Bakery Purchase

A bakery sells cookies at a rate of $$1.25$$ dollars per cookie. If a customer buys $$x$$ cookies, the total cost can be expressed as $$C = 1.25x$$. If the customer pays with a $$20$$ dollar bill and also uses a coupon that gives a discount of $$d$$ dollars, her total payment can be represented as:

$$P = 20 - (C - d)$$

What must be the value of $$d$$ for the payment to be exactly $$10$$ dollars after the transaction?