X Data Scientist/Software Engineer interview questions

Despite my mentioning that I had only heard about map-reduce at talks, and not really used it, he decided to ask me a question on the same subject. How would you solve this ML problem using map-reduce? Another question was, what type of problems would you be interested in solving given a large amount twitter data. These were essentially the two "substantive" questions that he asked me, which I presume he'd be using to judge my candidacy!