A caterpillar grows from a larva to an adult, doubles in size every day, and can grow to 40 cm in 20 days. How many days does it take to grow to 5 cm?
一条毛毛虫从幼虫长到成虫，每天长大一倍，20天能长到40厘米，当长到5厘米时，需用多少天．
<gadget id="calculator">20 - 1 - 1 - 1</gadget>
<output>17</output>

<result>17</result>