The area covered by the water lilies in the pond doubles every day, and it just covers the entire water surface in 40 days. How many days will it take to cover half of the water surface?
池中的睡莲所遮盖的面积每天扩大一倍，40天正好遮住整个水面，问遮住水面的一半需要多少天．
<gadget id="calculator">40 - 1</gadget>
<output>39</output>

<result>39</result>