I think these people mean "I can't do arithmetic" which is much more acceptable, in my opinion. If you asked me to find a derivative, I could probably do it faster than I could find 123*26. It's not because the multiplication is harder, but because I don't have to do it in my head that often anymore. I have a calculator, and I use it. However, I have to do derivatives in math class everyday and at this point I'm probably faster at it. Not being able to do arithmetic is fine by me, I can't do it.