I have to admit I wasnt very smart. The key word here is minimum. But I was thinking in terms of power of 2 perhaps to find out the answer and trying to put in arbitrary numbers to find out the different permutations. After analysing it for 2 days, I concluded I wasnt meant to be a software engineer at Facebook.

So i went to google out the answers on internet – the answer is 10.
Initially I thought that a higher or lower number will impact the probability of guessing the number. I have no idea why but now thinking back, I feel stupid. Going by halving the range each time, it does not matter what the number is, as long as the range is halved, it has maximized the probability of getting the final answer within the shortest time.