The water lilies in the pond grow at a rate of twice every day. On the 10th day, the whole pond is covered. May I ask how many days it is when half of the pond is covered?
池塘里的睡莲以每天两倍的速度增长，第10天长满整个池塘，请问长满半个池塘是第多少天．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>