There is a kind of water lily, which doubles in size every day, and can cover the whole pond after 8 days. How many days will it take to cover half of the pond?
有一种睡莲，每天长大1倍，经过8天可以把整个池塘遮住．那么遮住半个池塘需要多少天．
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<result>7</result>