During the past 15 or so years I have watched my son Andrew, who has Down Syndrome and is going to be 20 this year, grapple with the all too often abstract task of learning mathematics. Sometimes, it seems that no matter how many concrete examples we use he just cannot grasp a particular concept. The concept of comparison, for example, seemed to be completely beyond his ability to understand. At basketball games he cannot work out how much one team is winning or losing by even when the score is 4 - 2. It's not that he cannot do the calculation, he just cannot comprehend the idea of difference in a comparative sense.
I had pretty much given up with this particular concept when, purely by chance, a couple of months ago he looked at the underside of one of his Hotwheel cars and discovered it was first made the same year as he was born; 1993 was stamped into the underside. He was so excited to discover that it was the same age as he is that he now looks at each of his Hotwheels, a collection of some 300, to see if they are older or younger than he is. Success at last, his understanding of the concept of comparison is born but not because of anything I did but because he was motivated to understand. So I looked back on all the things he has learned; everything is related to his level of motivation. He bowled a perfect game on the Wii bowling because he was motivated, he can move a spinning basketball from finger to finger on one hand because he is motivated, he understands the idea of sequence because he is motivated by wanting to know what comes next.
Is he motivated to learn how to do the algorithm 45 - 21? Not at all, so he probably will not learn it (we stopped trying around 7th grade). He is, however, motivated to use a calculator which will work fine as long as he understands which operation to use; something he will be motivated to do as he shops more for himself and manages his credit card and bank account. Here's a presentation I gave about math and Down Syndrome at the Massachusetts Down Syndrome Congress last November.