What is a Guesstimate?
Well, as the name suggests, it is an estimate based on some intelligent guesses. There is no exact answer to a guesstimate.
What the interviewer is looking for is the approach taken by you. He/she is least interested in the correct answer or any numerical values involved.
What you have to prove to the interviewer is that while handling any problem:
Guesstimates are fairly easy to crack if you get a hang of them. So, the best possible way to master them is to practice as many as you can.
Some example guesstimates which were asked in various companies are given below: