In a situation of proportionality, the mathematical reasoning that makes it possible to determine a missing term when you know all of the other terms.

This proportional reasoning, gave rise to an algorithm called

**, which is easy to visualize and to apply.***cross multiplication*### Example

We used 8 litres of paint to repaint the walls of a room with a surface area of 40 m². How many litres of paint would we need to repaint the walls of a room with a surface area of 60 m²?

This arrangement leads to the equation:
40 |