So I am married to a math genius. He gets is from his parents, especially his dad. His dad's favorite bar trick back in the day was to tell people he could add or multiply faster in his head than you could with a calculator. By the time the person could get the numbers into the calculator, he would have the answer. He could supposedly go up to 17 digits (or 100 trillion) in his head.
Last night we were at the grocery store & while we were checking out, Ben goes "the total is going to be $114.25." Want to know what the total was, $117.55. He was off by $3.30. Subconsciously he managed to add everything we purchased as we walked through the store. The only reason he was off was because he was not sure how much all of the produce we bought weighed. He had estimated on the green beans, squash, zucchini, apples, sweet potatoes, avocado, and grapes. And yet was still within under $5. This is a very regular occurrence. He is normally within $5. Expect when we separate at the store & he does not see everything I put in the cart. Then he is off by $10-$15. Ridiculous, right?
Here's another even more ridiculous one... Easter weekend we went to the store where Ben stated we bought $200 worth of groceries. The bill totaled only $149. I was making fun of him for being so far off. I mean, he is never off by $50. So he shows me the receipt, where it showed how much we saved. We had saved $51. $149 spent + $51 saved = $200 worth of groceries. So not only did he add what we spent, he also managed to subconciously add how much we saved on every single item.
So my husband is seriously bizarrely smart when it comes to math. It kind of freaks me out every time he does it.