Intuitional mathematics

Oxymoronic to be certain and yet, I can not explain several things that have happened today. First, I got up this morning to head to town intending to put step one of my Pina Colada Free Escape into action which meant that I needed to cash a check. So I go to CheckNcash. I didn't pay attention to the check cashing fees when I signed on because I knew that it would always be cheaper than allowing a check to bounce. So I hand the clerk the check, tell her I want to load a debit card with 300.00 and I just stand around waiting for her to verify the check type.
While standing there I thought, "So that means I'll get about 80.00 back." Then I thought "Wait. Do I know that for sure?" So I squinted real good and read the fee chart that was too far away for me to see without glasses which means next trip to the DMV is FUBARed. 5% fee for cashing this kind of check. I struggled to figure this the "easy" way. "5 cents for every dollar. 400.00. So that's 400x.05. So that equals... um... uh... um... frak!"
Yea me that I could figure out 5% of a dollar is 5 cents. But WTF couldn't I figure out 5 times 400? So the clerk comes back, hands me the loaded card and says "that's 79.change back to you." In other words... 80.00. Shocked me to my core that I knew the answer in a split nano second iwthout thinking about it but gave up on the problem when I thought about it. Math Anxiety? I totally believe. But how did I know when I couldn't figure it out?
So the next thing that happens is that I am trying to wade through tons of irrelevent material for an egyptian google search, get pissed and head to the Wheatons blog to take my mind off of my frustrations. It worked. I laughed until I cried and the shop owner asked me what was so funny. I may have made a new convert. So then I start to blog when I notice Space.com posted an article about a long lost twin of ours. I thought wow, they found Nibiru.
But no its called Theia and the source for accurate information turns out to be gravity wells that new probes are going to observe. Cool. Well I click on the multimedia button to see what a lagrangian point is. I've never heard of it. Before the page came up, myself thought to myself... well duh, a gravity well is a place where an object is going to spin perpetually without a large space object in the middle of it. Sure enough the presentation shows 2 stable wells and 3 unstable wells. And that's what the thing was. An object caught in a gravity well will have a halo (eliptical) orbit and it will stay there until something larger and more compelling snatches it.
So now my question is, how is this happening? How can I know a math answer wihtout doing the work? How can I know about a lagrange point if I don't here Geordie or Sam explaining it to Picard or Jack? Let's face it, that's where most of my space info comes from. Or at least this is why I look things up.
And why am I having so much trouble with this Egypt stuff? I don't usually have problems with language. But then... I've never really been so fragmented in my studies.