Write a method to act as a cash register at a fast-food restaurant. The input parameter 'pay' indicates the amount of money the customer is giving you. The input parameters 'burger', 'fries', and 'soda' are 'true' if the customer is purchasing each of these items, 'false' otherwise. The costs are: burger is $5, fries are $3, and soda is $2. Return how much change the customer will get back. For example, if the customer pays $10 and only buys fries, he or she will get back $7.