The area of the water lily in the pond doubles every day, and if it takes 17 days, it will cover the entire pond. Just ask: How many days will it take for these water lilies to cover half of the pond?
池塘里睡莲的面积每天长大1倍，若经过17天就可长满整个池塘．试问：需要多少天，这些睡莲能长满半个池塘？
<gadget id="calculator">17 - 1</gadget>
<output>16</output>

<result>16</result>