For each customer, a yogurt shop charges a cents for the first 5oz of purchased yogurt. Each additional ounce of yogurt costs the customer b cents. What is the value of a ?
(1) A customer who buys 10oz of yogurt is charged 10 percent less per ounce than a customer who buys just 5oz of yogurt.
(2) A customer who buys 20oz of yogurt is charged $2.50.