Xiaoya plans to finish a 120-page book this week. She will read an average of 12 pages a day in the first three days, and how many pages a day in the next four days can she finish the book?
小亚计划本周看完一本120页的书，前三天平均每天看12页，后四天平均每天看几页才能将这本书看完？
<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">120 - 36</gadget>
<output>84</output>

<gadget id="calculator">84 / 4</gadget>
<output>21</output>

<result>21</result>