A kind of aquatic plant grows very fast and doubles every day. If you put 1 plant in the pond on the first day, the pond will be full on the 30th day. If you put 4 plants in the first day, how many days will the pond be full.
一种水草生长很快，一天增加一倍，如果第一天往池塘里投入1棵，第30天长满池塘，如果第一天投入4棵，多少天可以长满池塘．
<gadget id="calculator">30 - 1 - 1</gadget>
<output>28</output>

<result>28</result>