This question is more of an Intelligence Question rather than a Mathematics Question.
The water in the bucket doubles every minute. After 8 minutes the bucket is full. This means a minute earlier (i.e. at 7th minute) the water in bucket was half as compared to water in bucket after 8 minute.
So, if after 8 minutes bucket is full, after 7 minutes bucket will be half full. So the answer is 7 minutes.
