Age:
Aims:
To overcome difficulties with vertical subtraction.
Introduction:
We are all familiar with vertical subtraction. So we fail to understand the difficulties encountered by students of primary school when they first have to face it. For example take the following operation:
53
-19
Children can not understand why we borrow a ten and 3 units instead of 13. To us it seems obvious but if you think a little more about what happens but in fact we do not understand what we do. How can we overcome this difficulty?
Main part:
One first method is direct instruction (the teacher explains that students passively observe). The steps are:
A) Show them the number 53 represented with multicubes ( five tens and three units). Next to us there is a child who has several cubes in a box. We tell them that he is our neighbour.
B) We tell the children that we want to subtract 19 from 53 vertically.
C) We explain that the vertical subtraction begins with the units (they are already familiar with this because they have already been taught how to add). As we cannot subtract 3 from 9 we borrow 10 blocks from our neighbor, so the subtraction now becomes 13-9 = 4.
D) Then we start working with the tens. Now we subtract the ten that has to be removed anyway and another ten because we want to give it back to the neighbour.
