The** rule of three** is a useful mathematical tool that relates three known unknowns to find out the value of a fourth element, which establishes a relationship of proportionality between all the unknowns involved in this calculation.

The **online rule of three calculator** allows to find out the value of the unknown Y as follows:

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## How to make a rule of three

Although the formula leaves no room for doubt, **calculate a rule of three** is a frankly simple process that does not require very advanced mathematical knowledge. Our online tool will help you to save you a couple of mathematical operations, but if you ever have to calculate a rule of three manually, you can see that it does not involve any difficulty.

## Simple rule of three exercise

¿**What the rule of three is for** simple straightforward? Let's say we have to paint five rooms. If we know that with eight liters of paint we are able to paint two rooms, how much paint will we need for those five rooms? In this case, the operations to be carried out would be the following:

This example can be transferred to other series of everyday situations in which there is a **proportionality relation** between the variables involved in the mathematical problem. If this condition is fulfilled, we are in front of a very useful mathematical tool that, undoubtedly, we will use in more than one occasion, either to solve mathematical problems of the day to day or those that are proposed to us in the school or university.

## Rule of three in Excel

Excel does not have a specific function for **apply the rule of three** so we will have to create the formula ourselves.

As we have already seen the theory in the previous point, we are now going to learn **how to calculate a rule of 3** in the Microsoft spreadsheet program.

To do this, we open the program and write our proportionality relation in empty cells. We are going to use the following example:

If we earn 1,500 euros for working 45 hours, how much would we earn if we worked 30 hours?

To calculate this rule of three in Excel, we write the values of the statement in different cells and when we have it, we write the following formula:

=A3*B2/A2

Note that for the above formula you may have to modify the cell coordinates. It all depends on the references you have used to write the **proportionality relation necessary to calculate the rule of three**.

If you have any questions or problems solving this problem in Excel, leave us a comment and we will help you as soon as possible.

## The inverse rule of three

If you need it, we also have a inverse rule of three calculator for those exercises in which the unknowns maintain a relationship of inverse proportionality.

This means that we will use the inverse rule of three when **there is a decrease in variable B when the value of variable A increases**.

In the link we have left above you will find more information about this form of solving **inverse proportionality problems** in which there are three unknowns of which we know their value and there is a fourth unknown that we do not know how much it is worth.

Good afternoon, I would like you to please help me with a chemistry problem that I was left with as homework and I don't understand it. This is the problem:

In a farm 3400 acacia trees were planted in an area of 36 hectares; additionally a tomato crop was established where 7000 plants were managed per hectare in 3.5 hectares; near the acacia lot 3840 Saman trees were planted in 7.2 hectares.

how many hectares do i need to plant 3952 passion fruit plants?

Inf: 1 hectare = 10000 square meters

Planting distance 5.4 meters between plants.

I hope you can help me with this exercise