Total Cost with Discount Expression

Consider the equation that represents the total cost C of purchasing x items at a store, where each item has a fixed cost, plus a variable cost:

$$ C = 5x + 20 $$

In this equation, the fixed cost represents a base fee charged by the store, and the variable cost represents the cost that varies depending on the number of items purchased. If the total cost is reduced by 10% for purchases of more than 10 items, what will be the new expression for C if x is greater than 10?