Dyslexic arithmatic

The problem that some children have with Maths are not obvious. If we take, for example, a dyslexic child. These children sometimes say that they cannot add up a column of numbers because the numbers move about. Those of us who haven't experienced this phenomenon find it hard to understand quite what is happening within their brain, but it appears that their perception of the position of any given digit changes with time. So they perceive a number that is in the units column, to be temporarily in the tens column and vice versa. One solution to this is to use highlighter pens to highlight all the numbers in the units column yellow, all the numbers in the tens column red, all the numbers in the hundreds column blue... then the pupil does not have to add numbers based on their position, but on the basis of their background colours.