The water lily in a certain pond doubles in size every day, and can cover the whole pond in 28 days. Ask: How long does it take for the water lily to cover half of the pond?
某池塘的睡莲每天长大一倍，28天就能把整个池塘遮住，问：睡莲遮住半个池塘需要多少时间？
<gadget id="calculator">28 - 1</gadget>
<output>27</output>

<result>27</result>