The rule of three is a mechanism that allows Problem resolution linked to the proportionality between three known values ​​and a fourth that is a unknown. Thanks to the rule, the value of this fourth term can be discovered.

It is also important to be clear about other aspects of the aforementioned simple rule of three. We are referring to the fact that the problems it allows to solve are both direct and inverse proportionality. And this without forgetting either that to carry out one you must have three fundamental data: two magnitudes that are proportional to each other and a third.

In other words, a rule of three is a operation which is developed to know the value of the fourth term of a proportion from the values ​​of the other terms. According to its characteristics, it is possible to differentiate between the simple rule of three and the compound rule of three.

The simple rule of three is one that allows establishing the link of proportionality between two terms that are known (TO Y B) and, from the knowledge of a third term (C), calculate the value of the room (X).

Let’s see a example. A cook who, days ago, prepared three cakes with one kilogram of flour, now has five kilograms of flour and wants to know how many cakes he can make. To perform the calculation, apply the simple rule of three:

If with 1 kilogram of flour you made 3 cakes,
with 5 kilograms of flour he will prepare X cakes.

1 = 3
5 = X

5 x 3 = 1 x X
15 = X

In this way, the cook discovers that, with 5 kilograms of flour, you can prepare 15 cakes.

The simple rule of three can be direct or inverse. In the case of direct simple rule of three, the proportionality is constant: at an increase of TO, corresponds to an increase of B in the same proportion.

An example to understand this type of simple rule of three would be the following: in a store we want to buy some chairs and they tell us that they sell them in packs. Specifically, they tell us that 5 are worth 600 euros, but we need 8 and we want to know what the price is. Thus, to know the result we should carry out the following operations: 600 x 8 and the result, 4800, divide it by 5. Thus we would know that the eight chairs are worth 960 euros.

In the simple inverse rule of threeOn the other hand, constant proportionality is only preserved when, at an increase of TO, corresponds to a decrease of B.

An example to understand how the simple inverse rule of three works is this: today a freight company has floated three trucks to transport a certain number of packages on six trips each. However, yesterday, to move the same number of packages, there were only two trucks of the same dimensions and capacities. So we are faced with the question of how many trips did these two vehicles make?

To find out, the operation would consist of doing these steps: 3 x 6 and the result, 18, dividing it by 2, which would give us that the two trucks had to make 9 trips each.