There is a water lily in a pond that doubles in size every day and can cover the whole pond after 10 days. How many days will it take for water lilies to cover half of the pond?
有一个池塘中的睡莲每天长大一倍，经过10天可以把整个池塘遮住．问睡莲遮住半个池塘需要几天？
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>