和老爸聊天，他出了一个题，，，你的答案？

My dad has watched a variety TV program recently. He recalls that I even had

known the answer of one question when I was in my elementary school. 

The following is the question and what is your answer？

In a big lake， due to the heavy rain， the amount of water is doubled every day. On day thirtieth, the lake is full. On which day is the lake with half of the water？