I've heard two three past year stories. All of them said the same. One math and one eco (mostly micro) question. Choose one, and solve it in the time alloted. Then explain the solution to them. They might change some parameters to test your understanding.
All the best! When is your interview?
Yes, past year papers is also what I'm doing. Can't really do much academically other than revise. However, if they're asking questions like why ISI, who are you, why are you interested, etc., then it'll be weird. :-S