The area of the water lilies in the pond doubles every day. If it takes 17 days to cover the entire pond, how many days will these water lilies cover half of the pond?
池塘里的睡莲的面积每天长大一倍，若经17天可以长满整个池塘，多少天，这些睡莲能长满半个池塘．
<gadget id="calculator">17 - 1</gadget>
<output>16</output>

<result>16</result>