A small green onion doubles every day, 20 days to 36 cm, how many days does it take to grow to 9 cm.
一个小青葱每天长一倍，20天长到36厘米，长到9厘米用多少天．
<gadget id="calculator">20 - 1 - 1</gadget>
<output>18</output>

<result>18</result>