A caterpillar grows from a larva to an adult, doubles in size every day, and can grow to 20 cm in 40 days. How many days does it take to grow to 5 cm?
一条毛毛虫由幼虫长成成虫，每天长一倍，40天能长到20厘米，问长到5厘米时要用多少天．
<gadget id="calculator">40 - 1 - 1</gadget>
<output>38</output>

<result>38</result>