A kind of aquatic plant doubles every day and covers the entire pond in 30 days. Then, how many days is it when it grows to half of the water surface of the pond?
一种水草，每天长一倍，30天长满整个池塘的水面．那么，长到池塘水面的一半是第多少天．
<gadget id="calculator">30 - 1</gadget>
<output>29</output>

<result>29</result>