The water lilies in a certain pond double in size every day and cover the whole pond in 20 days. Ask: How many days will it take for the water lilies to cover half of the pond?
某池塘的睡莲每天长大一倍，20天就把整个池塘遮住，问：睡莲遮住半个池塘需要多少天．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>