Xiaohong reads an extra-curricular book. She reads 15 pages on the first day, and then reads 3 pages more than the previous day. On the last day, she reads 27 pages. 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读了3页，最后一天读了27页，她一共读了多少天？
<gadget id="calculator">27 - 15</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<result>5</result>