Question: Joe is looking at Barb, and Barb is looking at Matt. Joe is married, Matt is not. Is a married person looking at an unmarried person?
c.) Cannot be determined
If you said "c" — and most people do — you are incorrect. If you did get the incorrect answer take some time and figure out why the answer is wrong. Upon a modest amount of reflection it is pretty obvious that the answer is "a" (or "Yes"). This is because although we don't know Barb's marital status, under either scenario a married person would be looking at an unmarried one.
Often, unlearning requires that we don't accept the "obvious" solution or the first answer that pops into our head. Our intuition can sometimes be wrong and one strategy to protect us against our own follies is to take our time and think before answering.