Consider a cooking situation, where the recipe has the following instructions for us: for 2,5 cups of flour, add 2 tablespoons of oil. If we added 10 cups of flour, how much oil would we need? This problem is a typical example of a [b]proportion equation[/b]. In the equation, we have two quantities, where the [b]proportion[/b] (ratio) of the quantities is known. If the amount of one of the quantities is known, we can solve the amount of the other quantity (by denoting the unknown quantity by, for example, [math]x[/math], and solving [math]x[/math] from the equation).

Simple proportion equations have an algorithm that can be used for [br]obtaining the solution of the equation:[br][br]1. Cross multiplication: multiply the numerator of the first ratio with [br]the denominator of the second ratio, and also multiply the denominator [br]of the first ratio with the numerator of the second ratio. Both products[br] yield the same value, creating a new equation that is equivalent to the[br] original equation.[br][br]2. Divide the equation by the coefficient of the unknown variable.