A: Probably not nearly as much as you think.
Here is a humbling thought: a good percentage, perhaps even the majority, of everything you ‘know’ is wrong. Of course, this does not mean that your parents, teachers, or mentors lied to you. Instead, this fact is merely a consequence of progress within human civilizations. As the arrow of time flies forward, our knowledge base continues to expand as well as become more sophisticated.
Many of the facts, ideas, and theories that you use to make daily decisions will eventually be updated, discarded, or replaced. It is not that you operate with a large set of falsehoods. More often, much of what you know is simply incomplete and a better explanation will eventually be discovered. Also, some of our shared ‘common sense’ cultural assumptions about the world will turn out to be real howlers and become laughable nonsense for our descendants. Remember that such fictions as the earth-centered universe, the four elements (earth, water, air, fire), and the aether dominated western perceptions about ‘how the world really works’ for almost 2000 years.
Bottom Line: one must actively keep an open mind about other perspectives and worldviews. Consider the mantra ‘you may be wrong, you often are’. Work to fight your biases and attempt to see around constraints to your own thinking.