Xiaogang reads a book. He reads 10 pages on the first day, and then reads 5 more pages every day than the previous day. He just finished reading 40 pages on the last day. How many days did he study in total?
小刚读一本书，第一天读10页，以后每天都比前一天多读5页，最后一天读40页正好读完．他一共读了多少天？
<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<gadget id="calculator">30 / 5</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>