Kobayashi planned to complete a book of math exercises during the summer vacation. He planned to do 20 questions a day, but actually only did 16 questions a day. It took 5 days longer than planned. How many questions are there in this exercise book?
小林计划在暑假里完成一本数学练习题，计划每天做20道，实际每天只做了16道，结果比计划多用了5天，这本习题册一共有多少题？
<gadget id="calculator">16 * 5</gadget>
<output>80</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<result>400</result>