Fatty reads a story book. At the beginning, he reads 9 pages a day, and he finishes half of the book in 10 days. After that, he reads 15 pages a day, which happens to be finished within the borrowing period. How many days is the borrowing period for this book?
小胖看一本故事书，开始每天看9页，10天正好看完这本书的一半，以后他每天看15正好在借期内看完，这本书借期是多少天？
<gadget id="calculator">10 / 15</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">9 * (2/3)</gadget>
<output>6</output>

<gadget id="calculator">6 + 10</gadget>
<output>16</output>

<result>16</result>