Xiaolan needs to read a 600-page storybook. She has been reading it for 15 days, and there are 285 pages left. How many pages does Xiaolan read on average every day?
小兰要读600页的故事书，已经读了15天，还剩285页没有读，小兰平均每天读多少页？
<gadget id="calculator">600 - 285</gadget>
<output>315</output>

<gadget id="calculator">315 / 15</gadget>
<output>21</output>

<result>21</result>