Simple Rule of Three Definition

The rule of three is a mathematical operation that allows finding the fourth term in a proportion when you have three terms. The rule of three is said to be simple when there are two quantities involved in the problem posed. Let's take an example to illustrate this idea. Four pens cost ten dollars and we want to know the value of twelve pens. From these initial data, two parallel columns are formed: one of the pens and the other of their price (in which there is only one known price).

A precise formula

To solve this problem multiply by diagonal, that is, 12 x 10 and you get a total of 120 and then divide this amount by 4 and give a result of 30. Thus, we already have the price of the twelve pens that the problem posed ($ 30). As can be seen, it is a direct proportion problem, since the greater the number of pens, the higher their price.

Practical example

In the simple rule of three there are two different magnitudes that are multiplied and it is of a direct type because by increasing one

magnitude or variable the other is increased. This means that the rule of three could be inverse and not direct. Let's look at this inverse rule of three situation with another illustrative example. There are four workers who build a wall in twelve days and we want to know in how many days the wall can be raised with 6 workers.

Two columns of magnitudes are replaced (one for workers and the other for days). This rule of three is inverse because with more workers, fewer days are used to raise the wall, that is, the magnitudes are not directly proportional but are inversely proportional.

Thus, to solve the problem you have to multiply 4 x 12 (48) and divide the quantity by 6, which gives a result of 8; which means that with 6 workers they need 8 days to raise the wall.

Rule of three in everyday life

In this way, both the simple and direct rule of three and the inverse type facilitate a tool great math utility for daily life. It must be taken into account that the fundamental concept of this operation is the proportionality between two magnitudes, which we use in very different everyday circumstances: to calculate prices when buying, to solve problems with various magnitudes and their proportion or to to elaborate a plate of kitchen in which we handle quantities and proportions.

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