These interviews are designed to assess how quickly you can think on your feet and how you might approach complex problems. The interviewer tests how you logically come up with an answer, so you need to verbalize your thought process. The process is more important than the answer.
Proficiency at guesstimate and similar techniques are the absolute hallmark of the practical data analyst—the person who goes out and solves real problems in the real world. It connects a conceptual understanding (no matter how rough) with the concrete reality of the problem domain; it leaves no place to hide. Guesstimate also generates numbers (not theories or models) with their wonderful ability to cut through vague generalities and opinion-based discussions. For all these reasons, guesstimate is a crucial skill.
How many ping pong balls would fit in this room?
How many people are shopping on Snapdeal in India on a Friday at 3:00p.m.?
How many cars can pass through a toll plaza in a day?
How many engineers are needed each year in our country?
How many cars are sold each hour in India?