# A recipe calls for 5 cups of flour for every 6 teaspoons of salt. If you wanted to make a larger...

## Question:

A recipe calls for 5 cups of flour for every 6 teaspoons of salt.

If you wanted to make a larger batch of the same recipe with 15 cups of flour, how much salt would you need?

## Proportions and Variation:

We can establish a relation between two numbers that are proportional by means of a ratio {eq}x {/eq} and {eq}y {/eq}. That is to say, a proportion shows the similarity between two ratios. When two variables are dependent, variations in the magnitude of one variable will have a proportional effect on the other. When there is an increase or decrease of a variable {eq}x {/eq} with respect to another {eq}y {/eq}, for a ratio or constant K, variations are present. In the case that we have a direct variation, it happens that when one variable increases the other increases, which can also be written as: {eq}\frac{{{y_1}}}{{{x_1}}} = \frac{{{y_2}}}{{{x_2}}} {/eq}.

{eq}\eqalign{ & {\text{In this specific case }}{\text{,we have two proportional values }}\,x\,\left( {cups{\text{ }}of{\text{ }}flour} \right){\text{ and }} \cr & y\,\left( {teaspoons{\text{ }}of{\text{ }}salt} \right){\text{ that have a variation in directly proportional form}}{\text{. }} \cr & {\text{So we have:}} \cr & \,\,\,\,{x_1} = 5\,cups{\text{ }}of{\text{ }}flour \cr & \,\,\,\,{y_1} = 6\,teaspoons{\text{ }}of{\text{ }}salt \cr & \,\,\,\,{x_2} = 15\,cups{\text{ }}of{\text{ }}flour \cr & \,\,\,\,{y_2} = ?\,\,teaspoons{\text{ }}of{\text{ }}salt \cr & {\text{Since}}{\text{, }}x{\text{ and }}y{\text{ vary directly}}{\text{, then}}{\text{, when }}x{\text{ increases it also }} \cr & {\text{increases }}y{\text{. For this reason}}{\text{, it must be satisfied that:}} \cr & \,\,\,\,\frac{{{y_2}}}{{{x_2}}} = \frac{{{y_1}}}{{{x_1}}} \cr & {\text{So if we do cross - multiplying:}} \cr & \,\,\,\,{y_2} \cdot {x_1} = {y_1} \cdot {x_2} \cr & {\text{Now}}{\text{, solving for }}\,{y_2}{\text{:}} \cr & \,\,\,\,{y_2} = \frac{{{y_1} \cdot {x_2}}}{{{x_1}}} \cr & {\text{So}}{\text{, substituting the given values:}} \cr & \,\,\,\,{y_2} = \frac{{6 \times 15}}{5} = 18\,teaspoons{\text{ }}of{\text{ }}salt \cr & {\text{Therefore}}{\text{, you will need }}\boxed{18\,teaspoons{\text{ }}of{\text{ }}salt}{\text{ with 15}}\,cups{\text{ }}of{\text{ }}flour{\text{.}} \cr} {/eq}