So this was asked to me by one of my friend and is based on probability which I thing is Conditional \ Bayes Theorem.
You've 6 folders in your office where you store your documents (with equal probabilities). But in 1/3 cases - you simply leave the documents in your home. Today you need a document in your office & start searching your office folders. How many folders you'd have checked after which you can conclude that that the probability of finding that document in your office is 50%?