Ask most programming languages to add a tenth and two tenths and they will tell you, with complete confidence, 0.30000000000000004. They aren’t being careless. They count in twos, and in twos a tenth never comes out even; it repeats forever, the way a third does for us as 0.333…, so the machine keeps as much as it has room for and lets the rest go. We can’t write a third exactly either, and nobody calls us bad at arithmetic; we just learned not to mind. Money is where nobody will put up with it, which is why careful software doesn’t keep dollars at all. It keeps whole cents. This house does the same: its rent isn’t 221.08, it’s 22108, because a whole number is the one thing every way of counting agrees on.