Xiaohong reads a story book, and reads 15 pages on the first day, and then reads 3 pages more than the previous day every day, and finishes it in 5 days. How many pages does this storybook have?
小红看一本故事书，第一天看15页，以后每一天都比前一天多看了3页，5天看完．这本故事书共多少页？
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

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

<gadget id="calculator">15 + 12</gadget>
<output>27</output>

<gadget id="calculator">15 + 27</gadget>
<output>42</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">42 * (5/2)</gadget>
<output>105</output>

<result>105</result>