A worm doubles in size in one day, grows to 20 cm in 20 days, how many days does it take to grow to 5 cm.
一只小虫一天长大一倍，20天长到20厘米，长到5厘米需要多少天．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>