Answer:
It took 31 days for the patch to cover half the lake
Explanation:
The patch grows to double its size everyday
the patch completely covers the lake in 32 days
Since the patch doubles itself everyday, this means that the previous day before the 32nd day, the lake was just half covered.
Therefore, the the patch covered half the lake on the 31st day, i.e it took 31 days for the patch to cover half the lake