Xiaohong reads an extra-curricular book. She reads 15 pages on the first day, then reads 3 pages more than the previous day every day, and reads 36 pages on the last day. How many days did she read in total?
小红读一本课外书，第一天读了15页，以后每天都比前一天多度3页，最后一天读了36页，则她一共读了多少天．
<gadget id="calculator">36 - 15</gadget>
<output>21</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<result>8</result>