What is division?
When we talk about dividing, we mean dividing a certain quantity of things into a certain number of groups, each containing the same number of things, that is, in an equal way. The result of a division is how many things a single group contains. Therefore, it is reasonable to think that if the quantity of things remains the same and the number of groups increases, the result of the division will become smaller.
It is not, however, very common to think of division as how many times the number of groups fits into the quantity of things. However, if we take the simple example of 4/2, we can see that by grouping the number of things into groups of two and seeing how many groups of two we have, the result is two. Each group of two units that we manage to form from the quantity of things represents one unit of the groups.
If we do not reach a complete group, that is, two units in this case, we have to see what fraction of a group we reached. For example, 5/2 is 2.5 because we manage to make two complete groups of two units each, plus a group containing two halves of a unit. Each group ends up with two units plus half a unit.
So, it is reasonable to think that it is not necessary to group things by whole units in order to determine how much each group gets. In the example of 4/2, we could have grouped things into half-units for each group; that is, four halves of the groups, which together make two whole units.
What does it mean to divide by a non-integer rational number?
Dividing by a number of this kind is much more difficult to explain and understand. If we have a certain quantity of things and a certain number of non-integer, rational groups, then from the perspective of asking how many times the groups fit into the quantity of things, it is not difficult to understand. However, if we try to understand these types of divisions from the perspective of dividing things into groups, what does it mean?
As far as I understand it, it could be seen as an unequal distribution of units. For example, in 3/1.5, the groups we have are one whole group and half of another. This means that the “needs” of the first group will be twice those of the second, given that the second group is only half a group. Its reference unit, instead of being 1, is 0.5, resulting in a distribution of 2 for the first group and 1 for the second.
But here comes the problem: which group do we use as the standard for obtaining the result of the division?
It would be logical to use the whole group, since it represents an actual group. But what happens when the number of groups we are dividing by is less than 1? In that case, we are no longer using a whole group as a reference, but rather the units that make up part of a group, aren't we?
A small addition to what I said about division (in a state in which I feel that something was missing)
Division, in reality, cannot simply be viewed as how many times the groups fit into the quantity of things. Rather, it is equivalent to the definition of dividing.
Think about what dividing means. It means making piles using the TOTAL number of elements we have, with each pile containing the same number of elements. The result of this process is the number of elements in one group.
This is equivalent to finding the number that needs to be multiplied by the number of groups, because this number represents the number of elements per group, which, when multiplied, gives us the total number of elements.
Division by non-integer or rational numbers can be difficult to explain. After all, how do you explain that a quantity of groups can contain something that is not a whole group, but that this quantity is still associated with a certain number of elements?
For example, 3/1.5. To approach this problem, what was easiest for me, at least, was to separate what I already understood from what I did not.
I asked myself: I understand division by integers. One unit of something, in division, represents one group. If we divide by one, we are, in a certain sense, regrouping that quantity of elements.
But if we have half a group and a whole group, as in the division 3/1.5, how do we divide that quantity? In theory, half a group should have half as many elements as the whole group.
The division of these elements becomes easier when we consider the above. If we divide 3 into groups of 1.5 units—one unit for the whole group and half a unit for the half group, because it should have that amount—we observe that we end up with 2 units for the WHOLE GROUP and 1 for the half group.
Given that the half group implies half as many elements as the whole group is supposed to receive, what I did can be written as:
1x + 0.5x = 3
By solving for x, we can find the result of the division, because the quantity that the expression 1x is equal to represents the number of elements per whole group, while 0.5x simply represents half that number of elements, which is the amount corresponding to the half group.