Base ten system and its respective materials provide an effective dynamic model for addition that allow children to recognize the importance of adding ones to ones, tens to tens and so on. This model also is used to demonstrate the processes involved in regrouping. In the case of subtraction: when it is not possible to remove some ones from some ones, or some tens from some tens, or some hundreds from some hundreds etc. then exchange 1 ten for 10 ones and add to some ones, exchange 1 hundred for 10 tens and add to some tens, exchange 1 thousand for 10 hundreds and add to some hundreds etc., if necessary, in order to perform the subtraction of the digits. The use of such models is so important: they help children understand the procedures involved in addition and subtraction algorithms. Here is the geometrical representation of the addition operation (Dynamic Model) for 43 - 16 .

The model of this case and the respective procedure serve as a demonstrative model for the teachers to use in their classroom, equipped with computers, or in computer laboratories. By watching this case the students are able to figure out and build in their minds similar models for other cases of adding/subtracting multi-digit numbers. Manipulatives and visual aids are very important when teaching addition and subtraction. [br]_ Try to understand how the model is built.[br]_ Try to build your dynamic model for other cases of subtracting 2 other two-digit numbers.