It's a BULLSHIT question and you shouldn't answer it. What if they asked what the square root of 45,678,892 was? You would probably stop, look at them curiously, and say something like:

"What makes you ask?"

That's the same question you should ask in this moment as well. It's a bad strategy to answer a question for which you don't already know the "reason" behind the question. Sure -- there are some likely assumptions. But you don't KNOW why they are asking you.

SO ASK THEM THAT...

This isn't about honesty. Integrity is just a ticket to the game. This is about being a smart business person. You don't answer other random questions a potential client might ask you. Why blindly answer this one? Right?