An insect grows from a larva to an adult, doubles in size every day, and can grow 40 mm in 16 days. How many days does it take to grow to 20 mm?
一种昆虫，由幼虫长成成虫，每天长1倍，16天能长40毫米，问长到20毫米，需要多少天．
<result>15</result>