There is a water plant in the pool, and the water plant doubles in size every day, and the pool just fills up in 10 days. On the first few days, half of the pool is covered.
水池内有棵水草，水草每天都要长大一倍，10天正好长满水池．第多少天长满水池的一半．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>