Xiao Ming borrowed a 120-page story book for 9 days, and he read 30 pages in the first 3 days. From day 4 onwards, how many pages per day does he have to read on average to return it on time?
小明借了一本120页的故事书，借期9天，他前3天看了30页．从第4天起，他平均每天要看多少页才能按时归还？
<gadget id="calculator">120 - 30</gadget>
<output>90</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">90 / 6</gadget>
<output>15</output>

<result>15</result>