This sounds like a question used judge character. I'm not sure it makes sense to be doing such a thing at google, but sometimes these kinds of things are asked

This sounds like a question used judge character. I'm not sure it makes sense to be doing such a thing at google, but sometimes these kinds of things are asked more to judge your reaction rather than your answer. I'm sure you replied confidently, but others may get flustered, which could be a sign of how they act under pressure. And of course there are those who would answer take a best guess stab rather than appear to not know the answer which would be undesirable as well.