Xiao Ming reads a 210-page story book, and reads an average of 15 pages a day in the first 8 days. Read the rest within 9 days, how many pages must be read per day on average?
小明看一本210页的故事书，前8天平均每天看15页．剩下的在9天内看完，平均每天必须看多少页？
<gadget id="calculator">15 * 8</gadget>
<output>120</output>

<gadget id="calculator">210 - 120</gadget>
<output>90</output>

<gadget id="calculator">90 / 9</gadget>
<output>10</output>

<result>10</result>