"1) What is the relation between a Businss Analyst, Project Manager, Developer and Business Unit? How would you see yourself dealing with your colleagues?"

They work collaboratively as a team to achieve a desired result.

2) BA has one skill available all the time? List it

Interpersonal Communications ... You have to work across organizational levels (stake holders, developers, manager, etc) and negotiate between everyone who wants everything.

3) How many gallons of water is used in NYC? How did you come to that conclusion.

I figure an average person uses about 100 gal a day before / after work .. bath, shower, toilets, cooking, washing, etc.

Now, lets assume that person goes to work and uses the restrooms, water fountains, etc. several times a day and uses 50 gal during that time.

Plus other city stuff like washing cars, fire hydrant leaks ... etc. ... lets say 50 gal.

So, that is about 200 gal a day ... per person.

New York City (Remember - City and not State) has a population of about 8 million ... so 8,000,000 people x 400 gal per person) = about 1.6 billion gallons a day to start.

But since everyone does not work, take a shower / bath everyday or wash a car ... I'd subtract a several million of gallons from that.

So, My Answer would be about 1.3 - 1.4 Billion Gallons of water used daily in "New Your City" only.

How close am I ???

While I agree that making a good impression is important in an interview, the interviewer will be looking for a little more in response to these questions.

The first two questions are designed to see how well the interviewee understands the role of a Business Analyst and what qualities they need to posses. Along with any follow-up questions this will give an indication of how much experience the interviewee has in the role. Although there is not a single "right" answer there are a lot of key points that the interviewer will be checking for, such as understanding how the BA relates with the other roles mentioned in the question. The boundaries between them can vary between projects and organisations, so mentioning some odd projects you have worked on rather than just describing a single text-book scenario will indicate 'real life' experience (which rarely reflects the text books).

The third question is intended to see how well you break down a problem that at first appears impossible to solve, by applying logic, as the previous poster did. It doesn't matter whether the figures are correct or even if every use of water has been considered, but they will expect you to come up with a logical breakdown of the problem and be able to explain it effectively.