I still believe many who cannot do basic math can learn to use a simple calculator. But it is also true that many will not be able to learn the necessary sequence of button pushing. Adults learn differently from children, and one of those differences is that adults bring to the learning process a wealth of background and previous experiences, knowledge and experience of which the educator may not be aware.
Adults often have to unlearn first in order to relearn.
That's why adults often have a harder time with computers and foreign languages than children do. But if we assume right off the bat that someone cannot learn something because they can't do something else, we have already failed at helping adults reach the potential they all carry with them. Try color charts, try calculators, try graphs, and go ahead, try a slide rule. But don't expect to draw viable conclusions that a teaching technique will necessarily work from one adult to another, much less why this is true.