bacterium is cultivated in a laboratory, and its growth rate doubles every day compared with the previous day. Now put it into a bottle for cultivation, and grow to a full bottle in 20 days, then how many days will it grow to (1/4) bottle.
试验室培养一种细菌，每天比前一天增长1倍．现在放入瓶中培养，20天长到满瓶，那么第多少天长到(1/4)瓶．
<result>18</result>