A shopping mall opens at 8:00 in the morning and closes at 9:00 in the evening. What are the hours of operation of the store throughout the day?
一个商场上午8：00开时营业，到晚上9：00关门，这个商店全天的营业时间是多少小时．
<gadget id="calculator">21 - 8</gadget>
<output>13</output>

<result>13</result>