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

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

In other words, a rule of three is one **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 **simple three rule** and the **compound three rule** .

The simple rule of three is one that allows establishing the link of proportionality between two known terms (**TO** and **B** ) and, based on 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 a 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 three rule:

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

*1 = 35 = X*

*5 x 3 = 1 x X15 = X*

In this way, the chef discovers that, with

**5 kilograms of flour**can prepare

**15 cakes**.

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

An example to understand this type of rule of three simple would be the following: in a store we want to buy some chairs and they tell us that they sell them in pack. Specifically, they tell us that 5 are worth 600 euros, but we need 8 and we want to know what the price would be. Thus, to know the result we should perform 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 **reverse simple three rule** , on the other hand, constant proportionality is only preserved when, at an increase of **TO** , corresponds a decrease of **B** .

An example to understand how the rule of three simple reverse works is this: today a merchandise company has floated three trucks to transport in six trips each a certain amount of packages. However, yesterday, to move the same number of packages, there were only two trucks of the same size and capacity. So, we are asked the question of how many trips did those two vehicles make?

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