The hearing aid battery market has been quietly, but steadily, growing. This growth is attributed to an aging population and the improvement in hearing aid technology. Zinc air replacement batteries use air outside the battery as a source of power. There is a tab on the battery which seals the air holes to ensure freshness until the customer is ready to use the battery. To activate the battery, simply remove the tab, wait one minute to allow air to enter and activate the ingredients, and insert the battery into your hearing air. Zinc air batteries last 3 to 21 days, depending on the actual hearing aid and battery size. 

Costco is a major competitor in this market. In 2010 Costco reported 11.6 million zinc battery customers, who purchased a total of $ 1.58 billion of disposable hearing aid batteries. In the United States there were 48.5 million disposable hearing aid battery customers, who purchased $ 4.73 billion in this category. 

Calculate the heavy usage index for Costco customers. Report your answer rounded to two decimal places.