Xiaohua has been reading a 252-page storybook for 12 days, 15 pages a day, and has to finish the rest within 4 days. How many pages do she read on average every day?
小华看一本252页的故事书，已经看了12天，每天看15页，剩下的要在4天内看完，剩下的平均每天看多少页？
<gadget id="calculator">12 * 15</gadget>
<output>180</output>

<gadget id="calculator">252 - 180</gadget>
<output>72</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<result>18</result>