There are water lilies in a pond. It is known that the water lily doubles every day, it can cover the whole pond in 30 days, and half the pond can be covered in how many days.
一个池塘要种睡莲．已知睡莲每天成一倍增长，30天能够长满全池，多少天能长满半池．
<gadget id="calculator">30 - 1</gadget>
<output>29</output>

<result>29</result>