The duckweed in a pond doubles in size every day. In 20 days, the duckweed will cover the whole pond. On the number of days, the duckweed will cover (1/4) of the whole pond.
一个池塘浮萍每天长一倍，20天浮萍会长满整个池塘，第多少天浮萍能长满整个池塘的(1/4)．
<result>18</result>