I had almost exactly this same engineering test when google interviewed me in 2006. It was terrible, and left a bad taste in my mouth. Given the complexity of the work I was doing at the time, the entire thing seemed ridiculous.
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Recruiter: "I was looking for you to say it's a fingerprint"
So I guess I was wrong, because despite explaining them in decent detail, I didn't use the one keyword.
first thing I do is try to get them to ask it a different way. then I play spew out every related buzzword possible as if its a question. Are you talking about .... oh, you mean .... I can usually get it in a few tries.