Hi, 1. A yes-or-no problem is in P (Poynomial time) if it the answer can be computed in polynomial time. 2- A yes-or-no problem is in NP (Non-deterministic Poynomial time) if a yes answer can be verified in polynomial time. I don't understand here the difference between answering and verifying a problem in polynomial time. If we can verify that a yes-or-no problem is in NP in polynomial time, doesn't means that it can be answered?