Xiaohua and Xiaohong read the same book. Xiaohua reads 9 pages on the first day, and then reads 3 pages more than the previous day. As a result, she only needs to read 30 pages on the last day to finish reading. Xiaohong’s first day Read 15 pages, and then read 3 more pages every day than the previous day. As a result, you only need to read 12 pages on the last day to finish reading. How many pages does this book have?
小华和小红读同样的一本书，小华第一天读9页，以后每一天比前一天多读3页，结果最后一天只需要读30页就可以读完了，小红第一天读15页，以后每一天都比前一天多读3页，结果最后一天只需要读12页就可以读完，这本书共有多少页？
<gadget id="calculator">39 + 12</gadget>
<output>51</output>

<gadget id="calculator">9 - 12</gadget>
<output>-3</output>

<gadget id="calculator">30 + (-3)</gadget>
<output>27</output>

<gadget id="calculator">27 / 3</gadget>
<output>9</output>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">51 * 5</gadget>
<output>255</output>

<result>255</result>