A hair grows from larva to adult and doubles every day. It can grow to 40 centimeters in 20 days. How many days is it when it grows to 10 centimeters?
一条毛毛从幼虫到成虫，每天长大一倍，20天能长到40厘米，长到10厘米时是第多少天．
<gadget id="calculator">20 - 1 - 1</gadget>
<output>18</output>

<result>18</result>