Welcome to the Den, Sourabh. I have moved this thread to the appropriate forum section - New Logic/Math Puzzles.
This is one of my favorite puzzles. It is actually in the very first thread of the forum section mentioned above.
Let me direct your attention to the forum rules (see "Important: READ BEFORE POSTING" above). You will find many useful tips there - for instance, not to post puzzles which are already part of this site (you can do a quick search before you post a puzzle that you have not created).
Thread locked.