The binding workshop of the printing factory originally planned to bind 5,300 books, but 2,100 were bound in the first 7 days, and 400 remaining books were bound every day. How many days did it take to complete the task?
印刷厂装订车间原计划装订5300本书，开始7天装订了2100本，余下的书每天装订400本．完成任务共用了多少天？
<gadget id="calculator">5_300 - 2_100</gadget>
<output>3_200</output>

<gadget id="calculator">3_200 / 400</gadget>
<output>8</output>

<gadget id="calculator">8 + 7</gadget>
<output>15</output>

<result>15</result>