Liping Ma discusses the “concept” rather than the “procedure” of regrouping. For example, 12+9=21 students must see that there is more than 9 in the ones column so that ten must be put over the tens column to make 2 or 20. We must examine 2+9+11 and relate it to the problem. Also this is a great opportunity to show a member of the fact family, 21-12=9. In adding two-digit numbers, the basic step is to add the 10s together and add the 1s together. Sometimes this will produce more than 10 1s, in which case 10 of the ones should be grouped into a ten and then joined to the other 10s. In a subtraction problem, this procedure is reversed. If a 10 was formed during the addition, it will have to be broken up again to do the subtraction.
