The water lilies in a certain pond will double in size every day and can cover the whole pond in 15 days. How many days will it take when the water lilies cover half of the pond?
某池塘的睡莲每天会长大一倍，15天就能把整个池塘遮满，当睡莲遮住半个池塘的时，需要多少天．
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