kind of aquatic plant grows very fast, doubling in size every day. If you put one aquatic plant into the pond on the first day, it will grow into two plants on the second day, and four plants on the third day...the pond will be full on the 20th day. On the first few days, these aquatic plants just covered a quarter of the pond.
一种水草长得很快，每天增加一倍．如果第一天往池塘里投入一棵水草，第二天就长成两棵，第三天就长成四棵…第20天就长满池塘．第多少天的时候，这些水草正好长满池塘的四分之一．
<gadget id="calculator">20 - 1 - 1</gadget>
<output>18</output>

<result>18</result>