Ordinarily I would say that talking about science to people who believe the Book of Genesis is fact would be a pointless waste of time and thought, but I had sort of an epiphany arising from a little internal dialog I was running with an imaginary Amish person (I've been watching TV shows about the Amish, so that's not entirely random). This thought experiment was a lot of fun, and I enjoyed mentally having to break down science to its essence and communicate it to people who choose to avoid complex academic subjects. I don't put words in the mouth of the imaginary other, but rather establish a pattern of argumentation on my own part that is simple, understandable to anyone, and leads inexorably to the foundational essence of science. Now, that is not to say it would or could persuade someone who is robotic in their faith, but it would at least show sincerely devout people common sense on a level that is virtually universal.
I should emphasize that in practice, if it were to work at all, all of this would have to be said in a spirit of respect and light-hearted conversation, not as a lecture or sermon. In fact, a perfect opportunity to begin such a discussion is when a devoutly religious person who denies scientific understanding is proselytizing to you - at least if they themselves seem amiable rather than hostile.
1. After making clear that you want to start out by establishing some common ground, begin with a simple question: What is 1 + 1? If they are any kind of competent missionary for their beliefs, they will want to dispel what they view as miconceptions that believers are ignorant fools, so I wouldn't expect much evasiveness or rhetorical garnishment on this point - they'll just say 2.
2. Now ask them if God could make 1 + 1 be anything other than 2. Could God make 1 + 1 equal 3? Could he make it be 11? Could he make the answer be orange? Of course they will say Yes, and probably throw some Bible quotes at you.
3. Return to their response to the original question. When I first asked you what 1 + 1 is, you responded 2. Why? It could be anything at any time, right? You believe God can intervene in this matter, and routinely does intervene in the course of events, so why would you agree with the statement that 1 + 1 = 2 if it's not really settled?
4. They may become thoughtful at this point, but don't count on it. Most likely you will have to explain their own thinking to them: You said it was 2 because that is your experience and your common sense, despite the belief you've expressed that God could make it anything. When you count one and one, you have always gotten 2. Thus when asked by others what 1 + 1 is, you will say 2 as a matter of fact.
5. Extend the point a bit further, setting up an eventual contrast with how they respond to certain scientific subjects. If a child you were teaching wrote that 1 + 1 is 2 on a math test, you would not mark their answer wrong, I assume? And if their answer on a test was that 1 + 1 = Whatever God Says It is, would you mark that correct and give them the same credit as the child who marked 2? Or would you believe the child was just trying to hide the fact that they hadn't paid attention in class?
They might begin to hesitate at this point and sense that you may be leading things in a direction they don't want to go, so don't be surprised or distracted if they start to throw in tangential or even completely irrelevant Bible quotes as a smoke screen to break the momentum of your points - but at this point they aren't sure where you're going with it, so you should be able to get them to take a firm position on the question. If there is any spark of adult intelligence in their heads - and it is a fact, though baffling, that not all Creationists are morons - their answer will be that they would mark the answer 2 correct and the answer Whatever God Says It Is incorrect.
6. Appeal to their sense of authority, which is very strong among the religious. If it were you taking a math test and came across a very complicated question you didn't entirely understand, would you expect to receive credit for answering "God only knows"? Since they have already said they would mark such an answer wrong if they were a teacher, they will have to say No - or if they are dim enough to say Yes, you can point out the conflict with their earlier statement. Would you trust the teacher who says the answer is 4.5, even if you did not understand their explanation for why that's the answer? They will most likely answer Yes.
7. Now bring the point home. What if they tell you the equation you didn't understand describes the age of the Earth, and the 4.5 answer corresponds to billions of years? We've agreed that you are willing to set aside your religious beliefs in favor of your knowledge, reason, and experience when asked what 1 + 1 is. You are also willing to set them aside when dealing with purely abstract questions you don't understand or when asking someone more knowledgeable than you about it. So why are you not able to do that when literally every scientist in the world - people who know more math than most people can even imagine, and spend their entire lives using it to discover things about nature - tells you that the Earth is 4.5 billion years old, and that all living things including human beings developed through a process of evolution?
If 1 + 1 = 2 is correct in all practical terms despite your devout belief in divine intervention, how can you justify brushing off the conclusions of people with far more advanced skills than you? Why can't you acknowledge that insofar as 1 + 1 = 2, even though you believe it might not be, it is also just as correct to say that the Earth is 4.5 billion years old and all life evolves despite your faith in the Book of Genesis? You cannot have it both ways: Either it is blasphemous to say that 1 + 1 = 2, or it and every other conclusion that follows from human logic and scientific inquiry are perfectly compatible with your faith. If you can believe that God can make 1 + 1 = 3 and yet still understand and say that it equals 2, then you can believe in Genesis for whatever "spiritual truth" you see in it and still understand and acknowledge the facts of planetary formation and human evolution.
Obviously you're not going to convince any of these people right then and there, and they might politely disagree and end the conversation; or storm off in a huff; or become hysterical and start vomiting hellfire and brimstone verses at you. But I think any of them with even the dimmest spark of human intelligence flickering in their brains would remember and ponder your conversation for a long time to come, and in a few of them, additional thoughts would bloom.