A book, if you read 20 pages a day, you can finish it in 15 days. In fact, you read 120 pages in the first 4 days. At this speed, how many days will it take to finish the rest? (Solution with proportional knowledge)
一本书，如果每天看20页，15天可以看完，实际前4天看了120页，照这样的速度，余下的还要几天看完？(用比例知识解)
<gadget id="calculator">20 * 15</gadget>
<output>300</output>

<gadget id="calculator">300 - 120</gadget>
<output>180</output>

<gadget id="calculator">120 / 4</gadget>
<output>30</output>

<gadget id="calculator">180 / 30</gadget>
<output>6</output>

<result>6</result>